Pocillopora damicornis and its symbiotic dino¯agellates acquired ammonium from 2 to 10 lM solutions at rates (range: 5.1 to 91.8 nmol N cm)2 h)1) that were similar to those reported for symbiotic dino¯agellates of reefbuilding corals in previous studies (Muscatine and D'Elia 1978; Muscatine et al. 1979; Wafar et al. 1993). For P. damicornis from Hawaii, Muscatine and D'Elia, measured maximum uptake rates of ammonium that ranged between 3.57 and 9.21 lmol N chlorophyll a)2 h)1. Assuming a mean chlorophyll a concentration for Hawaiian P. damicornis of 4 lg cm )2 (actual range 2.0 to 6.5 lg cm )2; Muller-Parker et al. 1994), these rates convert to maximum uptake values ranging between 14.3 and 36.8 nmol N cm)2 h)1. These values compare well with the rates of ammonium uptake at 10 lM (range: 9.8 to 45.4 nmol N cm)2 h)1). The ammonium uptake by P. damicornis was not in¯uenced by the whether measurements were made in light or dark. Muscatine and D'Elia reported that the extent to which corals (P. damicornis) will take up ammonium in the dark was strongly in¯uenced by the length of the preceding period of photosynthetic activity. When the energy reserves from preceding periods of photosynthetic activity were deliberately run down in lengthy dark periods (>19 h), P. damicornis tended to show a net eux of ammonium. As the corals in the current study were collected directly from the experimental microatolls (1 m depth) in the early afternoon on a summer day, it appears that exposure of the corals to darkness during the current experiment was probably too small to aect the energy available for the transport of ammonium. The incubation of corals in ammonium did not aect the rate of ammonium uptake except in the case of the pink (pocilloporin) morph of Pocillopora damicornis measured at 10 lM ammonium. In this case, uptake waslower (P < 0.05) in corals exposed to elevated ammonium over 12 mo. The lack of a broad general response in this case ran counter to expectations from previous studies, which found a reduced ammonium uptake in symbiotic dino¯agellates isolated from P. damicornis grown for 6 wk in the presence of 20 lM ammonium (Yellowlees et al. 1994). Yellowlees and co-workers traced declines in ammonium uptake to reduced glutamine synthetase activities in the corals, which were exposed to increased ammonium. These decreases were interpreted as evidence of regulatory controls to reduce nitrogen in¯ux as excess inorganic nitrogen becomes available. The lack of a strong decreasing trend in the present experiment may be a result of the overall low loading of ammonium on the nutrient-treated microatolls. The addition of ammonium during the ENCORE experiment occurred only during the period in which the microatolls were ponded (3 to 4 h each low tide). The rapid uptake of ammonium in the microatolls during this period resulted in high concentrations of ammonium that only persisted for a couple of hours during each low tide (Koop et al. 1995). Counter to this explanation is the observation that P. damicornis growing in microatolls that received ammonium had low growth rates and higher rates of mortality than those growing in control microatolls (Hoegh-Guldberg 1999). This may indicate that the ammonium concentrations of the ENCORE project were higher than might be considered ``healthy'' for P. damicornis, and hence might be expected to induce biochemical controls on the in¯ux of nitrogen. These eects on growth and mortality, however, may also be related to secondary eects (e.g. great rates of algal overgrowth or disease infestation) as opposed to primary eects on coral/algal metabolism.