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Abstract-N-Serve blockage of NH,+ oxidation showed that nitrification in reef corals is quantitatively important. Average NO,-production was 9.4k6.0 nmol (mg coral tissue N) ' h-' (mean * 1 SE; n = 12), equivalent to 17% of the NH,+ consumption and 21% of the uptake by zooxanthellae showing an effective competition for NH,+ by nitrifiers. NH,+ utilization rates were equal to inorganic N production rates, and NO,-production rates were equal to NO,-uptake rates by zooxanthellae, suggesting close coupling between these processes. Density of nitrifying bacteria associated with living corals may vary from 4 to 260 x 10" cells (mg coral tissue N)-r.