Natural products are usually used as a food source for the culture of terrestrial molluscs. Better uniformity, and perhaps economy, could be introduced to the culture of molluscs by using synthetic diets. I assessed acceptance by various terrestrial molluscs of several insect diets developed for culture of caterpillars, and suitability for mollusc growth. Diet selection (preference) studies were conducted with 2 species of snails (Cuban brown snail, Zachrysia provisoria; Asian tramp snail, Bradybaena similaris) and 4 species of slugs (meadow slug, Deroceras laeve; grey fieldslug, Deroceras reticulatum; dusky arion, Arion subfuscus; Carolina mantleslug, Philomycus carolinianus). The molluscs differed in their preference for insect diets, though tobacco hornworm diet and a bean-based diet often used for cutworms usually were not preferred, whereas gypsy moth, diamondback moth, and spruce budworm diets tended to be more preferred. Short-term (7-wk) developmental (weight gain) studies were conducted using Z. provisoria, A. subfuscus, P. carolinianus and Florida leatherleaf, Leidyula floridana. These same diets demonstrated great differences in suitability for mollusc growth, and although one or more diets were suitable for most species, L. floridana performed poorly. Relative preference displayed by the molluscs for insect diets generally corresponded to performance on those diets. Long-term developmental studies were conducted for growth of Z. provisoria on gypsy moth, spruce budworm, and diamondback moth diets, and compared to development on a natural diet, Romaine lettuce. Snail weight gain followed a typical logistic growth curve over the course of the developmental period. Survival of Z. provisoria was high on all diets, and eggs were produced by snails developing on all diets. Zachrysia provisoria fed diamondback moth diet produced significantly heavier snails than those fed gypsy moth and spruce budworm diets, and weights equivalent to snails fed lettuce. This study demonstrates that some synthetic insect diets are suitable for growth of some terrestrial molluscs, but that preference and suitability vary among species.