Biological oxidation of radiolabeled NH(4) (half-life = 10 min) was observed within minutes in assays of an estuarine ammonium oxidizer and in natural populations of nitrifiers in coastal waters. Our estimates of turnover of the ammonium pool and rates of nitrification based on experiments using N are consistent with previous values in the literature based on longer-term N tracer experiments or on indirect methods and thus provide corroboration for the estimates by other researchers.