Intraspecific genetic variation is widely recognized to affect how bivalve larvae respond to variation in environmental conditions, but has been largely ignored in comparisons of larval performance between closely related sibling species. Replicates of five different full-sib families of larval blue mussels (Mytilus edulis and its more northerly congener, M. trossulus) were reared under different temperature (10, 13, and 17 °C) and food conditions in a full factorial design. Growth and survival were strongly affected by temperature, family, the interaction of family and temperature, and the three-way interaction of family, temperature, and food. Family means for days to 20 % survival ranged from 10.2 to 20.0 and did not cluster by species. The two M. trossulus families were intermediate in survival and ranked first and third for growth. The temperature by family interaction effect for survival was very strong, and the two M. trossulus families responded very differently to temperature manipulations. One M. trossulus family exhibited highest survival at 13 °C, while the other M. trossulus family and all three M. edulis families exhibited highest survival at 10 °C. By contrast, greatest growth consistently occurred at 17 °C, indicating that higher mortality in warmer water may be at least partially offset (at a population level) by more rapid growth in body size. The results illustrate the importance of assaying both growth and survival when evaluating environmental effects on larvae and the need to ensure a high level of genetic diversity when pools of larvae are used to study genetically similar species.