Phenotypic plasticity is the capacity of an organism's phenotype to vary in different environments. Although diet-induced phenotypic plasticity has been documented in New World cichlids, it has been hypothesised that this type of plasticity would be limited in certain Old World cichlids, because of the morphological constraints on the jaw imposed by mouth-brooding. This hypothesis was experimentally tested by determining the effect of different diets on the head and jaw morphology of split broods of several species of haplochromine cichlids from Lake Malaŵi, Africa, and two substrate-spawning cichlids, one from the Old World, Tilapia mariae (Boulenger), and one from the New World, Herichthys cyanoguttatum (Baird and Girard). Different feeding regimes resulted in differences in head morphologies in both New and Old World cichlid species. Although Old World mouth-brooding haplochromine cichlids exhibited phenotypic plasticity, the magnitude of head-shape plasticity observed was greater in the New World substrate-spawning cichlid, H. cyanoguttatum . The Old World tilapiine cichlid, T. mariae , did not exhibit phenotypic plasticity of head morphology. Experiments with modified foods demonstrated that the observed changes were unrelated to dietary nutrition, but were a result of differing feeding modes. Phenotypic plasticity might have contributed to the extensive trophic radiation and subsequent explosive speciation observed in Old World haplochromine cichlids. The existence of phenotypic plasticity has implications for morphology-based species descriptions as well.