Good day, everyone, and welcome to the Littelfuse Inc. First Quarter 2018 Conference Call. Today's call is being recorded. At this time, I would turn the call over to the President and CEO, Mr. Dave Heinzman. Please go ahead, sir.
Thank you, and good morning. Welcome to the Littlefuse first quarter 2018 conference call. Here with me, as always is Meenal Sethna, our Chief Financial Officer.
We're off to a great start in 2018, with first quarter performance significantly above our expectations. We are seeing broad-based demand across all of our businesses, and we delivered double-digit organic sales growth across all segments, including record sales for our IXYS business. We continued to leverage our bottom line and delivered strong cash flow for our quarter. We're making solid progress integrating the IXYS Corporation business and take an initial steps to drive synergy realization is, which I will discuss later in further details.
I am pleased with the consistent execution by our team as we continue to focus on the market of a safer, greener and more connected world. We remain on track with our strategy, and achieve our financial goal of double-digit sales and earnings growth combined with our strong balance sheet and the numerous growth opportunities across our circuit protection, power control sensor portfolios, we are well-positioned for the future. With that brief introduction, I'll turn the call over to Meena, who will provide additional color on the first quarter results.
Great. Thanks, Dave. Good morning, everyone. Let me start with some forward-looking statements. First during this call, certain comments will be -- we will make contain forward-looking statements, if forward-looking statements may involve significant risks and uncertainties, we ask you to review today's press release and our Form 10-K and 10-Q for more details about important risks that could cause actual results to differ materially from our expectations. Also our remarks today refer to non-GAAP financial measures. A reconciliation of these non-GAAP financial measures to the most comparable GAAP measure is provided in our earnings release and available on our Investor Relations website.
Now some highlights from our first quarter of 2018. We had an exceptional start to 2018 with sales growth much stronger than we expected, driving our sales and adjusted earnings per share well an excess of our January guidance. Sales in the first quarter were for $418 million, up 46% over last year, and up 10% organically. Notably, all of our segments had double-digit organic growth for the quarter. In addition, the IXYS business also stronger than our expectation with $86 million in sales for the 2.5 months in our portfolio and achieved a record $103 million in sales for the fourth quarter. GAAP operating margins finished at 9%. Our adjusted operating margin was 18.4%. We expanded adjusted margins 80 basis points over last year even with IXYS business results at margins lower than the company average.
Our adjusted operating income was up 53% over last year, as we continue to drive leverage through the P&L. We