provider customers, partially offset by negative transaction counts, comprised of negative transaction counts for DIY customers, partially offset by positive transaction counts for professional service provider customers. The improvement in average ticket values was the result of the increasing complexity and cost of replacement parts necessary to maintain the current population of better-engineered and more technically advanced vehicles. These better-engineered and more technically advanced vehicles require less frequent repairs, as the component parts are more durable and last for longer periods of time, creating pressure on customer transaction counts. However, when repairs are needed, the cost of replacement parts is, on average, greater, which benefits average ticket values. Average ticket values also benefited from increased selling prices on a SKU-by-SKU basis, as compared to the prior year, driven by increases in acquisition cost of inventory, which were passed on in market prices. Transaction counts prior to the stay at home orders taking effect were also negatively impacted by unseasonably mild winter weather in many of our markets, which did not stress vehicle components, resulting in a lower level of automobile parts breakage.
As the COVID-19 stay at home orders and recommendations took effect in our markets in the middle of March 2020, transaction counts for both DIY and professional service provider customers turned sharply negative, with a larger impact realized on the professional side of the business as we believe the demographic served by our professional service provider customers is more likely to accommodate working from home than a typical DIY customer. As a result of this impact from COVID-19, our comparable store sales for the four-week period beginning in the middle of March and extending into April decreased 13%. We cannot predict what continued impact the COVID-19 pandemic will have to our business in the future given that there is a high degree of uncertainty as to the duration and severity of the pandemic, the required necessary preventative stay at home measures and the mitigating impact of government stimulus for consumers.
We opened 73 net, new U.S. stores during the three months ended March 31, 2020, compared to opening 62 net, new U.S. stores for the three months ended March 31, 2019. In addition, on January 1, 2019, we began operating 33 acquired Bennett stores, and during the three months ended March 31, 2019, we merged of eight of these acquired Bennett stores into existing O’Reilly locations. As of March 31, 2020, we operated 5,512 stores in 47 U.S. states and 21 stores in Mexico compared to 5,306 stores in 47 U.S. states at March 31, 2019.
Gross profit:
Gross profit for the three months ended March 31, 2020, increased 1% to $1.30 billion (or 52.3% of sales) from $1.28 billion (or 53.1% of sales) for the same period one year ago. The increase in gross profit dollars for the three months ended March 31, 2020, was primarily the result of new stores and the acquired Mayasa stores and one additional day due to Leap Day, partially offset by the decrease in comparable store sales at existing stores. The decrease in gross profit as a percentage of sales for the three months ended March 31, 2020, was due to product mix, related to lower demand of seasonal products and a deleverage on fixed distribution costs as a result of the decrease in comparable store sales. We determine inventory cost using the last-in, first-out (“LIFO”) method, but have, over time, seen our LIFO reserve balance exhausted, as a result of cumulative historical acquisition cost decreases. Our policy is to not write up inventory in excess of replacement cost, and accordingly, we are effectively valuing our inventory at replacement cost.
Selling, general and administrative expenses:
Selling, general and administrative expenses (“SG&A”) for the three months ended March 31, 2020, increased 5% to $872 million (or 35.2% of sales) from $835 million (or 34.6% of sales) for the same period one year ago. The increase in total SG&A dollars for the three months ended March 31, 2020, was the result of additional facilities and vehicles to support our increased sales and store count and one additional day due to Leap Day. The increase in SG&A as a percentage of sales for the three months ended March 31, 2020, was principally due to deleverage on fixed costs as a result of the decrease in comparable store sales.
Operating income:
As a result of the impacts discussed above, operating income for the three months ended March 31, 2020, decreased 5% to $424 million (or 17.1% of sales) from $445 million (or 18.5% of sales) for the same period one year ago.
Other income and expense:
Total other expense for the three months ended March 31, 2020, increased 43% to $44 million (or 1.8% of sales) from $31 million (or 1.3% of sales) for the same period one year ago. The increase in total other expense for the three months ended March 31, 2020, was the result of a decrease in the value of our trading securities, as compared to an increase in the same period one year ago, and increased interest expense on higher average outstanding borrowings.
Income taxes:
Our provision for income taxes for the three months ended March 31, 2020, decreased 15% to $79 million (20.9% effective tax rate) from $93 million (22.5% effective tax rate) for the same period one year ago. The decrease in our provision for income taxes for the three months ended March 31, 2020, was the result of lower taxable income and benefits from renewable energy investment tax credits, partially offset by lower excess tax benefits from share-based compensation. The decrease in our effective tax rate for the three months