per pound by approximately $2.41. The increase in pounds sold was hindered in the quarter by a three-week unplanned outage of the 4-high hot rolling mill located in Kokomo, Indiana.
Cost of Sales. Cost of sales was $122.6 million, or 83.2% of net revenues, in the first quarter of fiscal 2024 compared to $109.6 million, or 82.6% of net revenues, in the same period of fiscal 2023. Cost of sales as a percentage of revenues in the first quarter of fiscal 2024 was higher than the first quarter of fiscal 2023 due to a three-week unplanned outage of our 4-high hot rolling mill.
Gross Profit. Gross profit was $24.7 million for the first quarter of fiscal 2024, an increase of $1.7 million from the same period of fiscal 2023. Gross profit in the first quarter of fiscal 2024 was higher than the first quarter of fiscal 2023 driven by higher volumes sold. Gross profit as a percentage of net revenues declined by 60 basis points, primarily due to the three-week unplanned outage of our 4-high hot rolling mill.
Selling, General and Administrative Expense. Selling, general and administrative expense was $12.5 million for the first quarter of fiscal 2024, an increase of $1.5 million from the same period of fiscal 2023. The increase in expense in the first quarter of fiscal 2024 as compared to the first quarter of fiscal 2023 was largely driven by $0.5 million of foreign exchange losses incurred during the quarter. Additionally, higher compensation and consulting costs contributed to the remainder of the increase.
Research and Technical Expense. Research and technical expense was $1.1 million, or 0.7% of net revenue, for the first quarter of fiscal 2024, compared to $1.0 million, or 0.7% of net revenue, in the same period of fiscal 2023.
Operating Income. The above factors resulted in operating income in the first quarter of fiscal 2024 of $11.1 million, approximately the same as the first quarter of fiscal 2023.
Nonoperating retirement benefit expense (income). Nonoperating retirement benefit expense (income) was a benefit of $0.5 million in the first quarter of fiscal 2024 compared to a benefit of $0.4 million in the same period of fiscal 2023. The higher nonoperating retirement benefit recorded was primarily driven by a decrease in the discount rate used in the actuarial valuation of the U.S. pension plan liability as of September 30, 2023, which resulted in higher amortization of actuarial gains in the first quarter of fiscal 2024 when compared to the first quarter of fiscal 2023. This was partially offset by a higher interest cost component of nonoperating retirement benefit income in the first quarter of fiscal 2024 when compared to the first quarter of fiscal 2023.
Interest expense. Interest expense was $2.2 million in the first quarter of fiscal 2024 compared to $1.5 million in the same period of fiscal 2023 primarily driven by approximately $30 million of additional borrowings against the revolving line of credit and higher interest rates.
Income Taxes. Income tax expense was $1.7 million during the first quarter of fiscal 2024, a decrease of $0.5 million from expense of $2.2 million in the same period of fiscal 2023. The decrease in income tax expense was primarily driven by the vesting of stock-based compensation awards that reduced tax expense by approximately $0.4 million during the first quarter of fiscal 2024 along with a decrease in income before income taxes of $0.6 million as compared to the first quarter of fiscal 2023. Income tax expense in the first quarter of fiscal 2024 as a percentage of income before income taxes was 18.2% as compared to 22.5% in the first quarter of fiscal 2023.
Net Income. As a result of the above factors, net income in the first quarter of fiscal 2024 was $7.7 million, relatively unchanged from $7.7 million in the same period of fiscal 2023.
Volumes and Pricing
Volume shipped in the first quarter of fiscal 2024 was 4.7 million pounds, which was 3.9% higher year-over-year than the first quarter of fiscal 2023, but was 2.8% lower sequentially than the fourth quarter of fiscal 2023. First quarter volumes are typically sequentially lower due to holidays, planned maintenance outages and customers managing their calendar year-end balance sheets. Additionally, during the first quarter of fiscal 2024, we had an unplanned three-week outage at our 4-high hot rolling mill which delayed production and shipments. This outage may impact shipments during the second quarter of fiscal 2024; however, a significant portion of the delayed shipments are expected to be made up in the second half of fiscal 2024 and into fiscal 2025.
Aerospace volume decreased by 1.5% during the first quarter of fiscal 2024 compared to last year’s first quarter; however aerospace average selling price per pound increased 15.4% resulting in a 13.7% or $8.8 million aerospace revenue increase