Local differentiation may occur during a short period of time and is part of the formation of new species. The isopod Asellus aquaticus is an example of a species in which local adaptation has occurred during a short period of time. An establishment of stonewort (Chara spp.) vegetation in Lake Tåkern (in the 2000) resulted in two different Asellus ecotypes; a lighter pigmented, smaller one that lives among stoneworts grazing periphytic algae, and a darker, larger ecotype that feeds on decaying leaves in reed (Phragmites australis vegetation. The purpose of this study was to examine whether there are differences in growth between ecotypes, depending on whether the food was periphytic algae or leaves, and to study the choice of substrates between the two food types. For the study, animals from both habitats were brought in from Lake Tåkern to the laboratory. I found that both the reed and the stonewort animals grew better when feeding on periphytic algae than on leaves, but that there was no difference in weight gain between the two ecotypes. There was no indication that the animals preferred any of the substrates. Results suggest that despite earlier noted differences in behavior, size and pigmentation (which differentiation had brought), there is no evidence that Asellus aquaticus has adapted to feed on plant matter prevailing in their original habitat. The animals grew better when the food was algae maybe due to that the algae, as previous studies show, are easier assimilated, whereas Asellus has to eat a larger amount of leaves to reach the same energy intake.