Stocking of trout into naturally fishless lakes in the mountains of western North America has reduced populations of many native species, particularly benthic aquatic invertebrates, in those systems. Although many bats consume emergent aquatic insects, no previously published studies have focused on how bats could be affected by changes to prey populations at lakes subsequent to trout stocking. The aim of this study was to determine whether fishless lakes or lakes stocked with trout provide higher quality foraging habitat for bats. We recorded and analyzed bat echolocation calls to assess overall bat activity, foraging activity, and foraging rates at nine feature-matched pairs of stocked and unstocked high elevation lakes in the central Sierra Nevada of California. Bats that echolocate around 40 kHz showed higher levels of overall activity, foraging activity, and foraging rates at stocked lakes. These higher activity levels could indicate the presence of higher quality foraging habitat. Alternatively, these bats could be foraging on suboptimal prey, pursuing small insects such as mosquitoes, and this could represent a cost to these bats due to the lower energetic return of small prey. Introduced trout may constitute a conservation issue to populations of bats in areas where both taxa occur.