This article revisits the Redfield ratio, a fundamental concept in aquatic biogeochemistry, which states that the elemental composition of plankton is 'uniform in a statistical sense'. The authors compile data on the elemental composition of marine phytoplankton from published studies to determine the range of C:N:P ratios. They find that the N:P ratio of algae and cyanobacteria is highly variable under nutrient-limited conditions, ranging from < 5 mol N:mol P when phosphate is abundant to > 100 mol N:mol P when inorganic nitrogen is present in excess. Under optimal nutrient-replete conditions, the cellular N:P ratio is more constrained, ranging from 5 to 19 mol N:mol P, with most observations below the Redfield ratio of 16. The critical N:P ratio that marks the transition between N- and P-limitation is estimated to be in the range 20–50 mol N:mol P, significantly higher than the Redfield ratio. The biochemical composition of phytoplankton can be used to constrain the critical N:P, suggesting that it lies between 15 and 30. Despite the average N:P composition of marine particulate matter closely approximating the Redfield ratio, there are significant local variations. The C:N ratio is also variable, but the average C:N ratio of nutrient-replete phytoplankton cultures is close to the Redfield value of 6:6. The authors conclude that the Redfield C:N:P ratio is not biochemically fixed and that the critical N:P ratio is likely influenced by environmental conditions, particularly irradiance.This article revisits the Redfield ratio, a fundamental concept in aquatic biogeochemistry, which states that the elemental composition of plankton is 'uniform in a statistical sense'. The authors compile data on the elemental composition of marine phytoplankton from published studies to determine the range of C:N:P ratios. They find that the N:P ratio of algae and cyanobacteria is highly variable under nutrient-limited conditions, ranging from < 5 mol N:mol P when phosphate is abundant to > 100 mol N:mol P when inorganic nitrogen is present in excess. Under optimal nutrient-replete conditions, the cellular N:P ratio is more constrained, ranging from 5 to 19 mol N:mol P, with most observations below the Redfield ratio of 16. The critical N:P ratio that marks the transition between N- and P-limitation is estimated to be in the range 20–50 mol N:mol P, significantly higher than the Redfield ratio. The biochemical composition of phytoplankton can be used to constrain the critical N:P, suggesting that it lies between 15 and 30. Despite the average N:P composition of marine particulate matter closely approximating the Redfield ratio, there are significant local variations. The C:N ratio is also variable, but the average C:N ratio of nutrient-replete phytoplankton cultures is close to the Redfield value of 6:6. The authors conclude that the Redfield C:N:P ratio is not biochemically fixed and that the critical N:P ratio is likely influenced by environmental conditions, particularly irradiance.