superior to, the financial information prepared and presented in accordance with GAAP and may be different from non-GAAP financial measures used by other companies, and therefore, may not be comparable among companies.
OVERVIEW
We are a developer, manufacturer and distributor of high-performance scientific instruments and analytical and diagnostic solutions that enable our customers to explore life and materials at microscopic, molecular and cellular levels. Our corporate headquarters are located in Billerica, Massachusetts. We maintain major technical and manufacturing centers in Europe and North America, and we have sales offices located throughout the world. Bruker is organized into two reportable segments, Bruker Scientific Instruments (BSI) and Bruker Energy & Supercon Technologies (BEST). Within the BSI Segment, we are organized into three operating segments: the Bruker BioSpin Group, the Bruker CALID Group, and the Bruker Nano Group.
Revenue for the three month period ended June 30, 2019 was $490.2 million, an increase of $46.5 million, or 10.5%, from the three month period ended June 30, 2018. Revenue from companies acquired within the past twelve months represented $38.6 million, or 8.7%, of the increase, and the unfavorable foreign currency translation effect of a stronger U.S. dollar relative to the Euro and other currencies represented a $13.3 million, or 3.0%, decline. Excluding these effects, organic revenue, a non-GAAP measure, increased by $21.2 million, or 4.8%. Revenues increased on an organic basis within the Bruker CALID Group and the Bruker BioSpin Group, and at the BEST Segment. From a geographic perspective, revenues increased in the North America, China and Japan regions.
Revenue for the six month period ended June 30, 2019 was $951.6 million, an increase of $76.2 million, or 8.7%, from the six month period ended June 30, 2018. Revenue from companies acquired within the past twelve months represented $64.5 million, or 7.4%, of the increase, and the unfavorable foreign currency translation effect of a stronger U.S. dollar relative to the Euro and other currencies represented a $33.0 million, or 3.8%, decline. Excluding these effects, organic revenue, a non-GAAP measure, increased by $44.7 million, or 5.1%. Revenues increased on an organic basis within the Bruker CALID Group and the Bruker BioSpin Group, and at the BEST Segment. From a geographic perspective, revenues increased in the North America, China and Japan regions.
Our gross profit margin increased to 47.0% during the three months ended June 30, 2019 compared to 46.2% for the three months ended June 30, 2018. Our gross profit margin increased to 46.8% during the six months ended June 30, 2019 compared to 46.2% for the six months ended June 30, 2018. The increase in gross profit margin in both periods resulted primarily from favorable product mix, operational improvements, accretive acquisitions and the positive impact of foreign currency translation.
Our operating margin decreased to 10.9% for the three months ended June 30, 2019 compared to 11.0% during the three months ended June 30, 2018. The operating margin decline was primarily due to assumption of expenses related to recent acquisitions. Our operating margin increased to 10.0% for the six months ended June 30, 2019 compared to 9.9% during the six months ended June 30, 2018. The operating margin expansion was primarily due to favorable product mix, operational improvements, accretive acquisitions and the positive impact of foreign currency translation.
Our income tax provision in the three month periods ended June 30, 2019 and 2018 was $10.6 million and $11.8 million, respectively, representing effective tax rates of 22.3% and 27.3%, respectively. The decrease in our effective tax rate was primarily due to the impact of a favorable discrete item in the period. Our income tax provision in the six month periods ended June 30, 2019 and 2018 was $18.3 million and $20.2 million, respectively, representing effective tax rates of 21.3% and 25.5%, respectively. The decrease in our effective tax rate was primarily due to the recording of items related to the update of a one-time tax on the mandatory deemed repatriation of post-1986 untaxed foreign earnings and profits (toll charge) assessed under the Tax Cuts and Jobs Act (2017 Tax Act) in the six months ended June 30, 2019, as well as a favorable discrete item.
Diluted earnings per share for the three month period ended June 30, 2019 were $0.23, an increase of $0.03 compared to $0.20 per share in the three month period ended June 30, 2018. Diluted earnings per share for the six month period ended June 30, 2019 were $0.43, an increase of $0.06 compared to $0.37 per share in the six month period