This conference call is also webcasted. It can be accessed at www.ametek.com and at www.streetevents.com. The conference call will be archived on both of these websites.
I will remind you that any statements made by AMETEK during the call that are not historical in nature are to be considered forward-looking statements. As such, these statements are subject to change based on various risk factors and uncertainties that may cause actual results to differ significantly from expectations. A detailed discussion of the risks and uncertainties that may affect our future results is contained in AMETEK's filings with the Securities and Exchange Commission. AMETEK disclaims any intention or obligation to update or revise any forward-looking statements. I will also refer you to the Investors section of www.ametek.com for reconciliation of any non-GAAP financial measures used during this conference call.
We will begin today with some prepared remarks, and then we will take your questions.
I will now turn the meeting over to Frank.Frank S. Hermance Thank you, Kevin, and good morning, everyone. AMETEK had an excellent first quarter. We established quarterly records for orders, sales, operating income -- operating margins, net income and diluted earnings per share. Sales in the first quarter were up 15% to $827.2 million. Internal growth was strong at 6%, while acquisitions added 10% and currency was a 1% headwind. Operating income for the first quarter was superb. It increased 20% to $182.8 million from $152 million last year, reflecting the impact of the higher sales and our Operational Excellence initiatives. Operating income margin in the quarter was a record at 22.1%, a 90-basis-point improvement over the first quarter of 2011. Net income was up 22% to $110.2 million and diluted earnings per share of $0.68 were up 21% over last year's first quarter. Orders in the first quarter were a record $863 million, up 8% overall from the prior year. Sequentially, orders were up 15%. The book-to-bill ratio in the quarter was 1.04.