The increasing occurrence of imposex in marine gastropods has been related to their exposure to the tri-substituted organotin compounds tributyltin (TBT) and triphenyltin (TPhT), which have been widely used as antifouling agents, and has been documented for more than 200 gastropod species. Odontocymbiola magellanica, a large benthic neogastropod, showed 85–100% imposex near harbours with high marine traffic in Patagonia. We evaluated if, as predicted by the energy allocation hypothesis, females with imposex (FWI) were on average larger and/or heavier than normal individuals, and if there were differences in shell morphometry associated with imposex, using both univariate and multivariate approaches. We detected differences in two morphometric variables, but no overall effect in shell shape, and ~10% of reduction in body weight in FWI. In addition, the multivariate index of size suggested a positive effect on FWI, in contrast to univariate analyses showing no differences in shell length. The present results demonstrate that responses to TBT vary among gastropod species and that the energy allocation hypothesis may be useful only under certain conditions.