The feeding habits of a population of stunted lake trout Salvelinus namaycush in Lake Louisa, Algonquin Park, Ontario, suggest that these fish are facultative planktivores in summer. Although other prey such as cyprinids and mayfly larvae were eaten, these fish fed mainly on zooplankton. They selected large cladocerans (99% of zooplankters eaten were Daphnia sp. longer than 0.9 mm), even though similar-sized copepods sometimes were more abundant in the lake. Electivities were similar for each depth stratum examined. We suggest the lake trout used all parts of the water column while foraging, and were not restricted to deep water as previously believed. However, periodic foraging excursions into the warmer summer epilimnion, low abundance of large prey, and gastric inefficiency may all contribute to the stunting of lake trout in this lake.