Lancashire won the toss and put Somerset into bat as the home side reached 103 for three at lunch in the County Championship clash at Taunton.
Eddie Byrom batted through the morning session for Somerset.
The 20-year-old left-handed opener was unbeaten on 38 at the interval, having shared stands of 39 with Marcus Trescothick and 46 with James Hildreth.
Lancashire surprisingly exercised their option to field first on a pitch expected to turn as the match progresses.
Trescothick, dropped before he had scored, progressed to 25 before falling lbw to Tom Bailey.
George Bartlett had made only three when leg-before moving across his stumps to Kyle Jarvis and Hildreth was bowled off an inside edge by Ryan McLaren, having progressed effortlessly to 25.