Planktivorous bluegill sunfish (Lepomis macrochirus) are known to consume large prey preferentially. Disagreement exists in the literature over whether this size-selectivity reflects optimal foraging behavior by the fish or relative prey encounter probability as predicted by a model in which apparent size of the prey determines foraging preference. As bluegills grow larger, their visual resolution increases, and this should affect prey choice under the apparent size model. In laboratory experiments, three sizes of fish were fed identical diets made up of three sizes of Daphnia magna. Large and medium-sized fish consumed relatively more large prey than predicted by the apparent size model, whereas the diet of small fish deviated only slightly from the apparent size model. For fish to be more selective than allowed by the apparent size model, they must be able to estimate absolute prey size. To do this requires good vision. Since diet selectivity was highly significantly correlated with the fishes' visual resolution in our experiments, we suggest that visual capability imposes a constraint on the ability of small bluegills to choose prey.