six months ended June 30, 2022 is due in part to the digestion of equipment at key accounts in early 2021 and better supply availability, which enabled us to partially fulfill product shipments against higher customer demand.
Sales in the Telecom and Networking market increased $6.1 million, or 18.9%, to $38.0 million for the three months ended June 30, 2022 and $8.1 million, or 12.4%, to $73.4 million for the six months ended June 30, 2022 as compared to the same periods in the prior year. The increase in sales was primarily due to improved material availability enabling us to better meet higher customer demand.
GROSS PROFIT
For the three months ended June 30, 2022, gross profit increased $27.1 million to $162.2 million, or 36.8% of revenue, as compared to $135.0 million, or 37.4% of revenue, in the same period in the prior year. For the six months ended June 30, 2022, gross profit increased $33.9 million to $306.5 million, or 36.6% of revenue, as compared to $272.5 million, or 38.2% of revenue, in the same period in the prior year. The decrease in gross profit as a percent of revenue is largely related to higher material and freight costs. Additional drivers of our decrease in gross profit as a percentage of sales include productivity inefficiencies resulting from supply constraints, COVID-19 capacity restrictions, and the transition of our Shenzhen, People’s Republic of China manufacturing to Penang, Malaysia. These factors were partly offset by increased volume and favorable product mix.
OPERATING EXPENSE
Research and Development
We perform R&D of products to develop new or emerging applications, technological advances to provide higher performance, lower cost, or other attributes that we may expect to advance our customers’ products. We believe that continued development of technological applications, as well as enhancements to existing products and related software to support customer requirements, are critical for us to compete in the markets we serve. Accordingly, we devote significant personnel and financial resources to the development of new products and the enhancement of existing products, and we expect these investments to continue.
Research and development expenses increased $7.9 million to $48.0 million for the three months ended June 30, 2022 and increased $11.3 million to $91.6 million for the six months ended June 30, 2022 as compared to the same periods in the prior year. The increase in research and development expense is related to the acquisition of SL Power, increased headcount and associated costs, outside technical services, and engineering materials as we invest in new programs to maintain and increase our technological leadership and provide solutions to our customers’ evolving needs.
Selling, General and Administrative
Our selling expenses support domestic and international sales and marketing activities that include personnel, trade shows, advertising, third-party sales representative commissions, and other selling and marketing activities. Our general and administrative expenses support our worldwide corporate, legal, tax, financial, governance, administrative, information systems, and human resource functions in addition to our general management, including acquisition-related activities.
Selling, general and administrative ("SG&A") expenses increased $6.9 million to $55.0 million for the three months ended June 30, 2022 and increased $9.5 million to $104.3 million for the six months ended June 30, 2022 as compared to the same periods in the prior year. The increase in SG&A is principally related to acquisition related activities, sales commissions driven by higher revenue, an increase in headcount, and an increase in variable compensation.
Amortization of Intangibles
Amortization expense increased $1.0 million to $6.5 million during the three months ended June 30, 2022 and increased $1.1 million to $12.0 million for the six months ended June 30, 2022, as compared to the same periods in the prior year. The increase is primarily driven by incremental amortization of newly acquired intangible assets. For