Eustrongylidessp. larvae were found to be numerous inBagrus docmac, Clarias mossambicusandHaplochromisspp. from Lake Victoria, rare inHaplochromisspp. and absent fromB. docmacandC. lazerafrom Lake George, East Africa. Larvae encyst in the mesenteries, spleen and inHaplochromisspp. also in the gonads. Unencysted larvae were observed in muscles. Cyst walls consisted of fibroblasts, collagen strands, and were often infiltrated with fat cells. Fatty changes were observed in infected spleens; the muscle tissue, around the unencysted worms, was digested and infiltrated with fibroblasts. Infected gonads, particularly ovaries were deformed and in heavy infections degenerated completely. The biological implication of this infection with larvalEustrongylidessp. on the host populations in Lake Victoria is discussed.