I found evidence that morphological plasticity is an important factor in the evolution of pumpkinseed sunfish ('Lepomis gibbosus') that are trophically polymorphic, inhabiting littoral and pelagic habitats within single lakes. Morphological variation in body form was induced by feeding different diet treatments and found to affect foraging performance in sunfish from one polymorphic population. A more extreme diet-induced pelagic morphology was associated with a significant increase in foraging performance in the pelagic source fish, while no effect was found in sunfish originating from a littoral source. To test for a relationship between observed morphological divergence in the wild and morphological plasticity, I estimated the degree of divergence and magnitude of plasticity in 6 populations of sunfish. A negative relationship was found between these two factors suggesting that morphological plasticity is found to a lesser degree in populations exhibiting a high degree of morphological variation. Together these findings suggest that morphological plasticity has likely played an important role in the adaptive divergence of these sunfish forms, and that selection probably has the capacity to act upon environmentally dependent phenotypes.