1. Specimens of a freshwater clam Anodonta were found to be symbiotized with zoochlorellae. Larger, hence older, clams showed a greater degree of infection with algae.2. In the light, clams exposed to the precursor NaH14CO3 showed a greater incorporation of 14C than corresponding control animals incubated in the dark. The difference is thought to result from the photosynthetic activities of the zoochlorellae.3. Chemical fractionation of selected host tissues revealed that in the light, 14C appeared in greatest amount in the small molecule fraction though it was also detected in other fractions.4. The major source of 14C in the hosts incubated in light is thought to be photosynthetic products translocated by the zoochlorellae. This view is strengthened by data showing that the symbionts can translocate to the medium when the algae are incubated in vitro.