The Yamaha International Corporation began first producing motorized products in 1955 with its YA-1 street bike. This motorcycle was manufactured and sold in Japan. By 1958, they were selling motorcycles in the United States through the distributor, Cooper Motors. Their first snowmobile, the SL350 was produced and sold in 1968. It was the first snowmobile to have slide-valve carburetors.
In 1976 Yamaha first produced its SRX model of snowmobiles. They quickly jumped to the front of the pack, taking the checkered flag from other brands of snowmobiles on the race tracks.
Below is a photo of a 1977 Yamaha 440 SRX snowmobile. The photo was taken at the 6th Annual Vintage Snowmobile Round Up in West Yellowstone in March of 2009.

Photography by Linda Aksomitis.
References:
Yamaha Motor Corporation. “Yamaha History Timeline, USA.” Accessed January/2010 from: http://www.yamaha-motor.com/corporate/historytimeline.aspx