Sea anemones (Aiptasia pulchella) fed 35S methionine labeled food translocated labeled material to symbiotic zooxanthellae. This uptake of host-derived organic materials by the symbiotic algae is interpreted as evidence for heterotrophy. It is calculated that 2.8-6.4% of the zooxanthellae growth requirement for protein was satisfied by heterotrophy under a 12 hour light:12 hour dark photoperiod. The cost of algal heterotrophy to the animal host was demonstrated by exposing aposymbiotic anemones to cultured zooxanthellae. Aposymbiotic animals rapidly re-established a stable symbiosis, but the symbiotic animals had a protein biomass significantly lower than control aposymbionts, suggesting that the reinfection of the symbiotic animals by zooxanthellae caused a decrease in animal biomass. Zooxanthellae spontaneously appeared and grew in the control aposymbionts. These repopulating algal cells were apparently derived from a residual heterotrophic population of zooxanthellae maintained by anemones after four y...