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 both Q3 and the first nine months of fiscal 2023, is a result of the strong sales 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 strong demand to continue through the balance of fiscal 2023 and into our next fiscal year. 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 our locking and access control 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 third quarter of fiscal 2023 generated record-breaking sales and profitability and I continue to believe that our best quarters are yet to come.”
Mr. Soloway concluded "We are excited to initiate our dividend program as the Company evolves and continues its strong growth patterns. NAPCO has created tremendous shareholder value over the years and the dividend program is another way for us to continue to do so in the future. We believe that it is important to balance our capital allocation priorities, including investing in growth opportunities, maintaining a strong balance sheet, and returning capital to shareholders."
Financial Results
Net sales for the quarter increased 21% to a quarterly record of $43.5 million, as compared to $35.9 million for the same period one year ago. Net sales for the nine months increased 25% to $125.3 million, as compared to $100.4 million for the same period one year ago. Research and development costs for the quarter increased 15% to $2.3 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 nine months increased 18% to $7.0 million or 6% of sales as compared to $5.9 million or 6% of sales for the same period a year ago. Selling, general and administrative expenses for the quarter remained relatively constant at $8.4 million or 19% of net sales, as compared to $8.4 million, or 24% of sales for the same period last year. Selling, general and administrative expenses for the nine months increased 3% to $24.7 million or 20% of net sales, as compared to $24.0 million, or 24% of sales for the same period last year.
Operating income for the quarter increased 160% to $11.9 million as compared to $4.6 million for the same period last year. Operating income for the nine months ended March 31, 2023 increased 185% to $28.6 million as compared to $10.0 million for the same period last year. Net income for the quarter was a quarterly record $10.8 million or $0.29 per diluted share as compared to $3.3 million or $0.09 per diluted share for the same period last year, a 231% increase. Net income for the nine months was $25.7 million or $0.69 per diluted share as compared to $12.1 million or $0.33 per diluted share for the same period last year, a 113% increase.
Adjusted EBITDA* for the quarter was a quarterly record $12.7, million, or $0.34 per diluted share, as compared to $5.7 million, or $0.15 per diluted share for the same period last year, a 123% increase. Adjusted EBITDA* for the nine month was $31.4, million, or $0.85 per diluted share, as compared to $13.5 million, or $0.37 per diluted share for the same period last year, a 132% increase.
Net income and earnings per share for last year's nine 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 nine months ended March 31, 2021 would have been $8.2 million and $0.22, respectively.
Balance Sheet Summary
At March 31, 2023, the Company had $56.9 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 $112.9 million at March 31, 2023 as compared with working capital of $93.1 million at June 30, 2022. Current ratio (defined as current assets divided by current liabilities) was 7.3:1 at March 31, 2023, and 4.5:1 at June 30, 2022.
Dividend Program
The Company also announced the initiation of a quarterly dividend program with the first dividend of $.0625 per share payable on June 12, 2023 to stockholders of record on May 22, 2023.
Conference Call Information
Management will conduct a conference call at 11 a.m. ET today, May 8, 2023. 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 8, 2023 and ending on May 15, 2023 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 13738574. In addition, the call will be webcast and will be available on the Company's website at www.napcosecurity.com.