School security continues to be a major concern to many of the 131,000 K-12's and 5,300 colleges and universities in the U.S. Despite the many shootings that continue to occur in schools throughout the U.S., the need for improved security measures is greater than ever, and increased federal and state funding continues to be available to aid most schools and school districts. As such, our fully integrated technologies for school security continues to remain a top priority for NAPCO and we remain focused on further penetrating this market.
NAPCO's outstanding record breaking results for the first half of fiscal 2023 is a result of the strong demand of each of our product lines including Starlink radios, our Alarm Lock and Marks locking products as well as our Continental access control systems, and we expect this demand to continue through the balance of fiscal 2023 and into next year. We are mindful of the unpredictable volatility associated with the current global supply chain constraints and we believe our seasoned management team's experience and methods from previous supply chain shortages position us well to mitigate such challenges. Our fundamental strategy continues to be to provide seamless security solutions for our customers and to continue to grow recurring service revenue, with both existing products as well as new ones, such as the unique to the industry Air Access products, which we expect will now generate a new stream of recurring service revenue from the locking and access control segments of our business. We continue to remain focused on our goal to generate strong revenue growth as well as increased profitability for the remainder of fiscal 2023 and beyond, thus delivering significant value to our shareholders. The second quarter of fiscal 2023 generated record-breaking sales and profitability and I believe that our best quarters are yet to come.
Mr. Soloway concluded, " Finally, I am pleased to announce that David Paterson, the Governor of New York from March 2008 until January 2011, has joined our Board of Directors. David has vast experience in crime and security issues and is an outspoken advocate of safety by combatting crime traditionally and with new methods and systems. He will bring his unique perspectives to NAPCO as we plan for the future growth and success of the Company".
Financial Results
Net sales for the quarter increased 27% to a quarterly record of $42.3 million, as compared to $33.4 million for the same period one year ago. Net sales for the six months increased 27% to $81.8 million, as compared to $64.5 million for the same period one year ago. Research and development costs for the quarter increased 12% to $2.2 million or 5% of sales as compared to $2.0 million or 6% of sales for the same period a year ago. Research and development costs for the six months increased 19% to $4.7 million or 6% of sales as compared to $3.9 million or 6% of sales for the same period a year ago. Selling, general and administrative expenses for the quarter decreased 5% to $7.8 million or 18% of net sales, as compared to $8.2 million, or 25% of sales for the same period last year. Selling, general and administrative expenses for the six months increased 5% to $16.3 million or 20% of net sales, as compared to $15.5 million, or 24% of sales for the same period last year.
Operating income for the quarter increased 643% to $9.4 million as compared to $1.3 million for the same period last year. Operating income for the six months ended December 31, 2022 increased 206% to $16.7 million as compared to $5.4 million for the same period last year. Net income for the quarter was a quarterly record $8.4 million or $0.23 per diluted share as compared to $1.0 million or $0.03 per diluted share for the same period last year, a 714% increase. Net income for the six months was $14.8 million or $0.40 per diluted share as compared to $8.8 million or $0.24 per diluted share for the same period last year, a 69% increase.
Adjusted EBITDA* for the quarter was a quarterly record $10.3, million, or $0.28 per diluted share, as compared to $3.1 million, or $0.08 per diluted share for the same period last year, a 232% increase. Adjusted EBITDA* for the six month was $18.6, million, or $0.50 per diluted share, as compared to $7.8 million, or $0.21 per diluted share for the same period last year, a 138% increase.
Recurring service revenue had a prospective annual run rate of approximately $61 million based on December 2022 recurring revenues. Based on January 2023, the prospective annual run rate for recurring service revenue dipped to approximately $59 million. Management believes this decrease is not reflective of any downward sales trend, but, instead is the result of an identifiable market event that may ultimately benefit the Company. Verizon, as previously announced, completed its 3G sunset in the beginning of January 2023, meaning that customers with 3G radios no longer received ongoing service. Management believes that a portion of the active NAPCO Starlink radios that lost communication due to the Verizon 3G sunset, have not yet been replaced. Ultimately, the Company anticipates that many of these NAPCO 3G radios will be replaced with NAPCO's newer generation radios which should result in both additional hardware revenue and increasing recurring revenue for the Company.
Net income and earnings per share for last year's six month period reflected Other income of $3.9 million which resulted from extinguishment of debt during the quarter ended September 30, 2021. Without such benefit, net income and earnings per share for the six months ended December 31, 2021 would have been $4.9 million and $0.13, respectively.
Balance Sheet Summary
At December 31, 2022, the Company had $47.1 million in cash and cash equivalents, other investments and marketable securities as compared to $46.8 million as of June 30, 2022. Working capital (defined as current assets less current liabilities) was $101.6 million at December 31, 2022 as compared with working capital of $93.1 million at June 30, 2022. Current ratio (defined as current assets divided by current liabilities) was 6.6:1 at December 31, 2022, and 4.5:1 at June 30, 2022.