Net income of $12.6 million, besides being a Q2 record breaker, represents 27% of our net sales. Adjusted EBITDA* of $15.1 million, also a Q2 record, represents an Adjusted EBITDA* margin of 32%.
Our balance sheet continues to get stronger, with cash and cash equivalents, other investments and marketable securities increasing 18% to $79 million as compared to $66.7 million at June 30, 2023, We have no debt and the net cash provided by operating activities for the six months ended December 31, 2023 was also strong, amounting to $18.7 million.
Our Alarm Lock and Marks locking hardware lines continue to see growth in school and classroom security, healthcare, and retail loss-prevention, as well as in multi-dwelling commercial and residential applications, growing approximately 10% compared to last year and approximately 18% compared to Q1. And locking sales once again represented 61% of hardware sales in Q2. We continue to remain focused on further penetrating each of these markets.
We recently announced the introduction of Prima by NAPCO, a new All-in-One Panel for security, fire, video and connected home. We anticipate that Prima will address an important mass segment of the security market, including residential and small business systems. With built-in Wi-Fi/cellular radio communications, customer alert notifications, and video and smart home subscription options for each -installed system, the security dealer, as well as the Company, can add more recurring-revenue generating accounts.
NAPCO's record-breaking results for Q2 of fiscal year 2024, was primarily the result of the continued growth and profitability from recurring revenues as well as the strong sales from our Alarm Lock and Marks locking product lines. Radio sales in Q2 improved over Q1, increasing by approximately 63%. While such sales were still 13% below the radio sales for Q2 last year, when the impending 3G Verizon sunset was rapidly approaching, the increase over last quarter was significant. We expect radio sales to continue to be a key contributor to our hardware sales and continue to lead to the continued growth of our highly profitable recurring revenues."
Mr. Soloway concluded, "We are halfway through fiscal 2024 and while we have already generated net income of $23.1 million, Adjusted EBITDA of $28.0 million and our Adjusted EBITDA margin is now 31%, there is more work to be done. While we are encouraged with the improved gross margin for hardware sales of 29%, we believe this could improve further as the fiscal year progresses. Our strong net income and Adjusted EBITDA* margins indicate the financial strength of our business. As such, we are pleased to continue our dividend program and we will be paying another dividend of $0.10 per share on March 22, 2024. This represents a 25% increase over the previously paid dividend on December 22, 2023 of $0.08. As always, we will strive to accomplish our goal of continued financial strength, product innovation, technical superiority, and strong profitability, for the rest of fiscal 2024 and beyond".
Financial Results
Net sales for the quarter increased 12% to $47.5 million (the highest quarterly sales in the Company’s history), as compared to $42.3 million for the same period one year ago. Net sales for the six months ended December 31, 2023 increased 9% to a six month record $89.2 million, as compared to $81.8 million for the same period one year ago. Research and development costs for the quarter increased 14% to $2.5 million, or 5% of sales, as compared to $2.2 million or 5% of sales for the same period a year ago. Research and development costs for the six months ended December 31, 2023 increased 7% to $5.0 million, or 6% of sales, as compared to $4.7 million or 6% of sales for the same period a year ago. Selling, general and administrative expenses for the quarter increased 11% to $8.7 million or 18% of net sales, as compared to $7.8 million, or 18% of sales for the same period last year. Selling, general and administrative expenses for the six months ended December 31, 2023 increased 5% to $17.1 million or 19% of net sales, as compared to $16.3 million, or 20% of sales for the same period last year.
Operating income for the quarter increased 219% to $13.8 million as compared to $4.3 million for the same period last year. Operating income for the six months ended December 31, 2023 increased 218% to $25.4 million as compared to $8.0 million for the same period last year. Net income for the quarter increased 221% to a quarterly record $12.6 million, or $0.34 per diluted share, as compared to $3.9 million, or $0.11 per diluted share, for the same period last year and represents 27% of net sales. Net income for the six months ended December 31, 2023 increased 229% to a six month record of $23.1 million or $0.62 per diluted share as compared to $7.0 million or $0.19 per diluted share for the same period last year and represents 26% of net sales.
Adjusted EBITDA* for the quarter increased 191% to a quarterly record $15.1 million, or $0.41 per diluted share, as compared to $5.2 million, or $0.14 per diluted share for the same period last year and equates to an Adjusted EBITDA* margin of 32%. Adjusted EBITDA* for the six months ended December 31, 2023 increased 182% to a six month record $28.0 million, or $0.76 per diluted share, as compared to $9.9 million, or $0.27 per diluted share for the same period last year and equates to an Adjusted EBITDA* margin of 31%.
Balance Sheet Summary
As of December 31, 2023, the Company had $79.0 million in cash and cash equivalents, other investments and marketable securities as compared to $66.7 million as of June 30, 2023. Working capital (defined as current assets less current liabilities) was $128.5 million at December 31, 2023 as compared with working capital of $111.7 million at June 30, 2023. Current ratio (defined as current assets divided by current liabilities) was 7.1:1 at December 31, 2023, and 6.7:1 at June 30, 2023.