Gastropods commonly show enormous variation in growth rate, even among identically reared siblings. This was investigated using a small isolated population of Deroceras laeve (Müller) — a species with low genetic variability. A high degree of variation in growth rate was evident, even among offspring from unmated mothers. This confirmed our hypothesis that this variation does not require high genetic polymorphism. Four additional hypotheses concerning the causal mechanism(s) for this variation were investigated: (1) Maternal influence through variation of egg quality was rejected as a possible cause for the variation since animals from mothers raised on either high- or low-quality diets did not differ significantly in growth or maturation rates (when egg size was controlled). (2) The possibility that animals might have their growth trajectory fixed by early nutritional experience was tested by initially rearing slugs on either high- or low-quality diets, and then reversing their food. Such "nutritional imprinting" was not supported. (3) Intraspecific interaction among individuals was not supported as a cause of variation either since stunted slugs did not show improved growth when isolated from conspecifics, and there was equal variation among individuals reared from the egg in isolation. (4) Variation in the egg size was the only characteristic investigated that significantly changed rates of juvenile growth and the timing of maturation. Larger eggs produced slower-growing slugs that matured later, and egg size seemed to account for the full range of observed variation. The ultimate function of the mechanism remains to be determined, but possibilities include adjusting the performance of offspring to resource supply, ensuring availability of mature individuals to breed during favourable microclimates, reducing competition among members of a cohort, or ensuring sexual heterogeneity for hermaphroditic breeding.