$125,339,000 as compared to $100,364,000 for the same period a year ago. The increase in sales for the nine months ended March 31, 2023 was due primarily to revenue increases in recurring communication services ($10,544,000), Napco brand intrusion products, which include the Company’s cellular radio products ($1,993,000), Alarm Lock brand door-locking products ($9,045,000), Marks brand door-locking products ($2,212,000), and Continental brand access control products ($1,182,000). The Company’s increase in equipment sales was primarily due to a general increase in demand for the Company’s hardware products.
The Company's gross profit increased by $6,246,000 to $21,279,000, or 48.9% of net sales, for the three months ended March 31, 2023 as compared to $15,033,000, or 41.9% of net sales, for the same period a year ago. Gross profit on equipment sales was $7,610,000, or 26.8% of net equipment sales, for the three months ended March 31, 2023 and $4,539,000, or 19.0% of net equipment sales, for the same period a year ago. Gross profit on service revenues was $13,669,000, or 90.3% of net service revenues, for the three months ended March 31, 2023 and $10,494,000, or 87.2% of net service revenues, for the same period a year ago. The increase in gross profit in dollars and as a percentage of net sales on equipment sales and service revenues was primarily the result of the increase in revenues of each as described above as well as increased availability and lower costs of components and transportation as compared to the same period last year, which resulted from improvements within the Company’s supply chain. The increases in revenues resulted in improved overhead absorption rates. In addition, the increase in gross margin on service revenues was due, in part, to increased service revenues relating to the Company’s fire radios, which have higher monthly selling prices than the Company’s intrusion radios.
The Company's gross profit increased by $10,267,000 to $50,199,000, or 40.1% of net sales, for the nine months ended March 31, 2023 as compared to $39,932,000, or 39.8% of net sales, for the same period a year ago. Gross profit on equipment sales was $11,170,000, or 13.7% of net equipment sales, for the nine months ended March 31, 2023 and $11,003,000, or 16.4% of net equipment sales, for the same period a year ago. Gross profit on service revenues was $39,029,000, or 89.1% of net service revenues, for the nine months ended March 31, 2023 and $28,929,000, or 86.9% of net service revenues, for the same period a year ago. The increase in gross profit in dollars on equipment sales for the nine months ended March 31, 2023 was primarily the result of the increase in revenues as described above, which improved overhead absorption rates, increased availability and lower costs of components and transportation as compared to the same period last year as well as a favorable shift in product mix the Company’s Alarm Lock brand door locking products, which typically have higher margins. These factors were mostly offset by the lower margins realized during the first two quarters of fiscal 2023 which resulted, in part, The decrease in gross profit as a percentage of equipment sales was primarily the result of the sale of the remaining portion of finished goods that were in opening inventory that contained certain higher priced components as disclosed in the Form 10-Q/A for the period ending September 30, 2022. The Company purchased these components at a significant premium during the supply chain interruptions during the latter part of fiscal 2022 in order to continue to supply the Company’s communication devices that led to the creation of recurring service revenues for the Company. The effect of these higher-priced components on the nine months ended March 31, 2023 was partially offset by the higher sales volume during this period as compared to the same period a year ago. The increase in gross margin on service revenues was due primarily to increased service revenues relating to the Company’s fire radios, which have higher monthly selling prices than the Company’s intrusion radios.
Research and development expenses for the three months ended March 31, 2023 increased $305,000 to $2,314,000, or 5.3% of net sales, as compared to $2,009,000, or 5.6% of net sales, for the same period a year ago. Research and development expenses for the nine months ended March 31, 2023 increased $1,046,000 to $6,964,000, or 5.6% of net sales, as compared to $5,918,000, or 5.9% of net sales, for the same period a year ago. The increase in dollars was due primarily to salary increases and additional staff.
Selling, general and administrative (“SG&A”) expenses for the three months ended March 31, 2023 remained relatively consistent at $8,425,000 as compared to $8,442,000 for the same period a year ago. SG&A expenses as a percentage of net sales decreased to 19.4% for the three months ended March 31, 2023 as compared to 23.5% for the same period a year ago. The decrease as a percentage of net sales was due primarily to the increase in net sales without the need to increase to increase SG&A expenses. SG&A expenses for the nine months ended March 31, 2023 increased by $736,000, or 3.1%, to $24,719,000 from $23,983,000 for the same period a year ago. SG&A expenses as a percentage of net sales decreased to 19.7% for the nine months ended March 31, 2023 as compared to 23.9% for the same period a year ago. The increase in dollars resulted primarily from increases in expenses relating to the Company's President and Chairman and the Company’s Executive Vice President and Chief Financial Officer selling shares of our Common stock in an underwritten secondary public offering, which is discussed more fully in Note 11 to the Condensed Consolidated financial statements, and credit card processing fees related to our monthly recurring service revenues. The decrease as a percentage of net sales was due primarily to the increase in net sales as partially offset by the aforementioned increase in expense dollars.