A compilation of data on the elemental composition of marine phytoplankton from published studies was used to determine the range of Cratio Nratio P. The Nratio P ratio of algae and cyanobacteria is very plastic in nutrient-limited cells, ranging from <5 mol Nratio mol P when phosphate is available greatly in excess of nitrate or ammonium to >100 mol Nratio mol P when inorganic N is present greatly in excess of P. Under optimal nutrient-replete growth conditions, the cellular Nratio P ratio is somewhat more constrained, ranging from 5 to 19 mol Nratio mol P, with most observations below the Redfield ratio of 16. Limited data indicate that the critical Nratio P that marks the transition between N- and P-limitation of phytoplankton growth lies in the range 20–50 mol Nratio mol P, considerably in excess of the Redfield ratio. Biochemical composition can be used to constrain the critical Nratio P. Although the biochemical data do not preclude the critical Nratio P from being as high as 50, the typical biochemical composition of nutrient-replete algae and cyanobacteria suggests that the critical Nratio P is more likely to lie in the range between 15 and 30. Despite the observation that the overall average Nratio P composition of marine particulate matter closely approximates the Redfield ratio of 16, there are significant local variations with a range from 5 to 34. Consistent with the culture studies, lowest values of Nratio P are associated with nitrate- and phosphate-replete conditions. The highest values of Nratio P are observed in oligotrophic waters and are within the range of critical Nratio P observed in cultures, but are not so high as to necessarily invoke P-limitation. The Cratio N ratio is also plastic. The average Cratio N ratios of nutrient-replete phytoplankton cultures, oceanic particulate matter and inorganic N and C draw-down are slightly greater than the Redfield ratio of 6·6. Neither the analysis of laboratory Cratio Nratio P data nor a more theoretical approach based on the relative abundance of the major biochemical molecules in the phytoplankton can support the contention that the Redfield Nratio P reflects a physiological or biochemical constraint on the elemental composition of primary production.