Production and recovery of ammonia-N was studied in the second and third trophic levels of a mariculture system on St. Croix, US Virgin Islands. The diatom Chaetoceros curvisetus, grown on nutrients in artificially upwelled deep water from 870 m depth, was the food source for Tapes japonica. Consumption of the phytoplankton by T. japonica increased throughout the day and decreased at night, and was related to corresponding changes in algal culture density. Feeding efficiency was highest at night. Ammonia-N production by the clams fluctuated over a typical 24 h period; dropping during the day and increasing at night. The ammonia-N concentration in the shellfish tank effluent was inversely related to the quantity of phytoplankton consumed by the clams. At all ration levels the small clams produced ammonia-N at a greater rate than large clams. H. rnusciformis fragmented and washed out of some tanks; the fragmentation was related to high ammonia-N concentrations in the inflowing water. High light intensity and temperature alone do not appear to cause fragmentation, but may have induced a trace nutrient deficiency in H. musciformis grown in the ammonia-rich seawater. Where fragmentation was not obvious, ammonia-N uptake per g H. musciformis was highly correlated with the average ammonia-N concentration of the inflowing seawater both day and night. Percent uptake of ammonia-N increased with increasing concentration of the nutrient in the inflowing seawater, reaching a plateau of about 70 % uptake of the available ammonia-N at concentrations above 4 ~g-at/1.