quarter as compared to the same quarter last year, indicates that security equipment professionals are getting increased access to both commercial and residential installation sites.
Our fully integrated technologies for the school security market continues to remain a top priority. The COVID-19 pandemic has caused some delay in spending at certain K-12 schools, colleges, and universities, but with the country starting to open up again, including schools at all levels, we are beginning to see more activity. While we have seen postponements of planned security upgrades, we have not seen a significant number of cancellations.
With our country now reopening at an accelerated pace, the results of our investments in R&D should become more evident. In the last eighteen months we have launched several new major products and solutions, including (1) iSecure, our all-inclusive cellular alarm system which offers the dealer the lowest upfront equipment cost in the industry, easy installation, fast programming and a quick return on investment, and (2) Air Access®, our new cellular, cloud-hosted access control product line which will allow access control integrators and locking professionals to build a recurring revenue business for themselves, just like our burglar and fire alarm dealers do. While still in the very early stages, we expect both of these products to provide the Company the opportunity to generate recurring service revenue from each of our divisions: alarms & connectivity, locking and access control."
Mr. Soloway concluded, "Our record-setting revenue and profitability in Q3, along with continued strong recurring revenue margins, gives us tremendous momentum as we head into Q4, historically our strongest quarter. I am proud of how the NAPCO team responded to the challenges brought on by COVID-19 and of our resiliency and ability to execute even during difficult and uncertain times. We remain focused on generating continued strong revenue growth as well as increased profitabilty. We will continue our efforts to expand our recurring revenue product offerings into all segments of the Company. As we look to the balance of fiscal 2021 and beyond, NAPCO is well positioned for long-term growth and profitability expansion."
Financial Results
Net sales for the three months ended March 31, 2021 increased 8% to a third quarter record of $28.2 million, as compared to $26.2 million for the same period one year ago. Net sales for the nine months ended March 31, 2021 increased 0.3% to $78.6 million, as compared to $78.4 million for the same period a year ago. Research and development costs for the quarter were $1.9 million as compared to $1.8 million for the same quarter a year ago and were 7% of sales for each of the quarters ended March 31, 2021 and 2020. Research and development costs for the nine months ended March 31, 2021 were $5.7 million as compared to $5.4 million for the same period a year ago and were 7% of sales for the nine months ending March 31, 2021 and 2020, respectively. Selling, general and administrative expenses for the quarter decreased 2% to $6.0 million, or 21% of sales, as compared to $6.1 million, or 23% of sales for the same period last year. Selling, general and administrative expenses for the nine months ended March 31, 2021 decreased 3% to $18.0 million, or 23% of sales, as compared to $18.6 million, or 23.7% of sales for the same period last year. Operating income for the three months ended March 31, 2021 increased 24% to $5.0 million as compared to $4.0 million for the same period a year ago. Operating income for the nine months ended March 31, 2021 was $11.4 million as compared to $11.6 million for the same period a year ago, a 3% decrease. Net income for the three months ended March 31, 2021 increased 21% to $4.4 million, or $0.24 per diluted share, as compared to $3.6 million, or $0.20 per share, for the same quarter last year. Net income for the nine months ended March 31, 2021 decreased 5% to $9.9 million, or $0.54 per diluted share, as compared to $10.4 million, or $0.56 per share, for the same period last year.
Adjusted EBITDA* for the three months ended March 31, 2021 increased 13% to $5.5 million, or $0.30 per diluted share, as compared to $4.9 million, or $0.26 per diluted share for the same period last year. Adjusted EBITDA* for the nine months ended March 31, 2021 was relatively constant at $12.9 million, or $0.70 per diluted share, as compared to $13.0 million, or $0.70 per diluted share for the same period last year.
Balance Sheet Summary
At March 31, 2021, the Company had $34.1 million in cash and cash equivalents and marketable securities as compared to $18.2 million as of June 30, 2020. Working capital (defined as current assets less current liabilities) was $70.0 million at March 31, 2021 as compared with working capital of $61.0 million at June 30, 2020. Current ratio (defined as current assets divided by current liabilities) was 5.5:1 at March 31, 2021 and 4.5:1 at June 30, 2020.
Conference Call Information
Management will conduct a conference call at 11 a.m. ET today, May 10 ,2021. Interested parties may participate in the call by dialing 1-877-407-4018 or for international callers, 1-201-689-8471, about 5-10 minutes prior to the start time of 11 a.m. ET. The conference call will also be available on replay starting at 2 p.m. ET on May 10, 2021 and ending on May 17, 2021 at 11:59 p.m. ET. For the replay, please dial 1-844-512-2921 domestically, or 1-412-317-6671 for international callers, and use the replay access code 13716033.
In addition, the call will be webcast and will be available on the Company's website at www.napcosecurity.com.