At September 30, 2021, we had no debt outstanding and no draws on the Revolving Facility other than $0.4 million of letters of credit which count as draws against the available commitment under the Revolving Facility. We believe that, absent a significant acquisition, cash on hand and cash flows from operations will be sufficient to meet our operating needs, ongoing capital needs, including current and possible future modernization, expansion, and development projects, and liquidity needs and allow us to pay regular quarterly cash dividends for the near future.
Results of Operations.
Revenues in the third quarter 2021 were $52.3 million, compared to $43.7 million in the third quarter 2020, an increase of $8.6 million, or 19.6%. For the first nine months 2021, revenues were $143.1 million, compared to $119.7 million in the first nine months 2020, an increase of $23.4 million, or 19.6%. Revenues from our lime and limestone operations in the third quarter 2021 increased $8.3 million, or 19.0%, to $51.7 million from $43.5 million in the third quarter 2020. For the first nine months 2021, revenues from our lime and limestone operations increased $22.8 million, or 19.2%, to $141.8 million, compared to $119.0 million in the first nine months 2020. The increase in lime and limestone revenues in the third quarter were primarily due to an 18.5% increase in sales volume, compared to the third quarter 2020, principally due to increased demand from our construction, steel, industrial, environmental, and oil and gas customers. In addition to the added Carthage revenues, the increase in lime and limestone revenues in the first nine months 2021, compared to the first nine months 2020, resulted primarily from increased sales volume of 17.1% principally due to increased demand from our construction, steel, environmental, and roofing customers. Revenues also included $0.6 million and $1.3 million from our natural gas interests in the third quarter and first nine months 2021, respectively, compared to $0.3 million and $0.7 million in the comparable 2020 periods, respectively.
Gross profit was $17.3 million and $45.9 million in the third quarter and first nine months 2021, respectively, compared to $14.2 million and $34.4 million in the comparable 2020 periods, increases of $3.2 million and $11.5 million, or 22.3% and 33.5%, respectively. Gross profit from our lime and limestone operations in the third quarter and first nine months 2021 was $17.1 million and $45.6 million, respectively, compared to $14.3 million and $34.8 million in the comparable 2020 periods, increases of $2.9 million, or 20.1%, and $10.8 million, or 31.1%, respectively. The increases in gross profit in the 2021 periods, compared to the comparable 2020 periods, resulted primarily from the increased revenues discussed above and increased operating efficiencies, partially offset by higher fuel costs. Gross profit also included $213 thousand and $307 thousand from our natural gas interests in the third quarter and first nine months 2021, respectively, compared to losses of $74 thousand and $386 thousand in the third quarter and first nine months 2020, respectively.
Selling, general and administrative (“SG&A”) expenses were $3.1 million in the third quarter 2021, compared to $2.9 million in the third quarter 2020, an increase of $0.2 million, or 8.1%. SG&A expenses were $9.2 million in the first nine months 2021, compared to $9.0 million in the first nine months 2020, an increase of $0.2 million, or 1.8%. The increase in SG&A expenses in the third quarter 2021, compared to the third quarter 2020, was primarily due to increased personnel expenses, including stock-based compensation. In the first nine months 2021, compared to the first nine months 2020, the increased personnel expenses were partially offset by lower legal expenses in the first nine months 2020, compared to higher legal expenses in the first nine months 2020, which were largely related to the acquisition of Carthage in the first nine months 2020.
Interest expense was $63 thousand in the third quarter 2021, compared to $62 thousand in the third quarter 2020, and $187 thousand in the first nine months 2021, compared to $186 thousand in the first nine months 2020. We had no outstanding debt during any of the periods. Interest and other income, net was $147 thousand and $272 thousand in the third quarter and first nine months 2021, respectively, compared to $49 thousand and $400 thousand, respectively, in the comparable 2020 periods.
Income tax expense was $3.0 million and $7.4 million in the third quarter and first nine months 2021, respectively, compared to $1.9 million and $4.7 million, respectively, in the comparable 2020 periods. Our effective income tax rate was reduced from the federal rate primarily due to statutory depletion, which is allowed for income tax purposes and is a permanent difference between net income for financial reporting purposes and taxable income.
Our net income was $11.3 million ($1.99 per share diluted) in the third quarter 2021, compared to net income of $9.3 million ($1.65 per share diluted) in the third quarter 2020, an increase of $2.0 million, or 21.3%. Net income in the first nine months 2021 was $29.4 million ($5.19 per share diluted), an increase of $8.5 million, or 40.4%, compared to net income of $21.0 million ($3.72 per share diluted) in the first nine months 2020.