Experiments were done to determine if ammonium, phosphate and feeding on Artemia nauplii affected the population density of symbiotic algae (zooxanthellae) in the Red Sea coral Stylophora pistillata. Corals were incubated for 14 days under natural sunlight at reduced intensity in running seawater aquaria. The seawater was continuously spiked to give final concentrations of either 20 $\mu$ M ammonium or 2 $\mu$ M phosphate, or both. A second set of similarly treated corals was also fed Artemia nauplii daily. Population density of zooxanthellae in corals spiked with ammonium, or ammonium plus phosphate, approximately doubled, and the ratio of zooxanthellae carbon. nitrogen decreased. Phosphate supplementation alone had no effect. The increase in zooxanthellae numbers was linearly proportional to the increase in protein in zooxanthellae, suggesting that availability of inorganic nitrogen leads to increased protein synthesis in zooxanthellae. Feeding on Artemia alone or together with phosphate had no effect on the population density of zooxanthellae. Feeding on Artemia and ammonium produced a small increase in population density but it was not statistically significant. The small effect could be due to insufficient influx of ammonium in fed animals, or growth of both animal and algae resulting in little or no net change in the population density of zooxanthellae. The results are consistent with the hypothesis that the growth of zooxanthellae in S pistillata from the Red Sea is nitrogen limited.