The effect of a change in the environment on reproductive and somatic energy allocation in an iteroparous freshwater clam Anodonta piscinalis was studied at different time of the seasonal reproductive cycle. Environmental change was produced by reciprocal transplant experiments among sites of varying productivity. In addition, clams were caged at high density to reduce the availability of resources. Transplanting females before fertilization (from May to June), or during the early development of the brood (from July to August) had no detectable effect on reproductive output. Early-season environment, however, affected body mass and percent fat content of females from two populations. This suggests that maintaining the level of reproductive allocation when resources are reduced early in the season leads to lower allocation to somatic growth and biochemical storage. Transplanting females late in the season (from September to November) had a substantial effect on reproductive output and body mass, but not on fat content, suggesting that late in the season allocation to biochemical energy storage is important. Hence, late in the season reproductive allocation may be adjusted to prevailing conditions in preparation for winter. Indeed, over-wintering site had a significant effect on percent fat content, body mass and shell growth when females were kept in a new environment from September to March. Variation among transplant sites in female body mass matched the estimated productivity of the sites, suggesting that it occurred in response to differences in the productivity of the habitats. The results emphasize the importance of taking seasonal changes in the priorities of energy allocation and the seasonality of reproductive processes into account when developing or testing models of optimal energy allocation.