Examination of data from literature showed that non-inclusion of urea uptake in nitrogen uptake by phytoplankton results in statistically significant overestimates of f. The extent to which f was overestimated because of non-inclusion of urea uptake depended upon the ecosystem type and was 6, 17, 24, 42 and 55%, respectively, in upwelling, coastal, polar, oceanic and estuarine waters. Systematic urea uptake measurements thus become necessary, but where such data are lacking, corrections are possible with the ratio between the f values calculated with and without urea uptake for each ecosystem.