Abstract The potentially molluscivorous cichlid fish Astatoreochromis alluaudi is known to exhibit a pronounced phenotypic plasticity in its pharyngeal jaw apparatus. Two phenotypes (wild-caught snail-eating specimens and specimens raised on soft food) were examined for differences in the number, size, shape, spacing and wear of functional teeth on the lower pharyngeal jaw. During growth, snail-eating specimens maintain tooth numbers but invest in teeth of a larger size (width and depth). In contrast, specimens fed a soft diet invest in more teeth, their size remaining unchanged except for the central, most posterior teeth. All changes in the dentition must be achieved through successive tooth generations. Serial microradiographs in the caudal area of the lower pharyngeal jaw, a region that is most significant in food processing, indicated that functional teeth in hard-food specimens more often show a successor below. This may be due to more time needed for larger replacement teeth to form and possibly to a shorter replacement cycle linked to the greater wear of the functional teeth. It is hypothesized that maintenance of tooth numbers and increase of tooth size in hard-food specimens is achieved by a one-for-one replacement and expansion of the tooth-bearing region and possibly by closer spacing of the teeth. Increase of tooth numbers in the soft-food specimens is probably achieved through the establishment of new tooth loci at the margins of the dentigerous area in addition to a one-for-one replacement.