The relationship between the f -ratio [NO 3− uptake/(NO 3− + NH 4+ ) uptake] and ambient nitrate concentration was evaluated for eight data sets from coastal waters. The f -ratio increased asymptotically with increase in nitrate concentration in most data sets. However, the rate at which f -ratio increased at low nitrate concentration (slope = m) and the maximum attained f -ratio (f max ) varied among regions; the initial slope varied most with values ranging in excess of an order of magnitude. The data were analyzed in relation to environmental factors and methodological considerations known to influence the f -ratio. Ambient ammonium concentration was important in accounting for regional differences in the f versus NO 3− relationship. A further analysis of the data, relating f -ratio to the ratio of NO 3− /(NO 3− + NH 4+ ) concentrations yielded a much more regionally consistent and approximately linear relationship; slopes varied by less than a factor of two in the extreme cases. Inclusion of known alternative (aside from NH 4+ ) sources of reduced-N (e.g. urea) and correction for methodological/computational errors (isotope dilution) systematically reduce f -ratio estimates. Other factors, e.g. reduced-N uptake by microheterotrophs, may systematically increase the f -ratio.