The thesis that the minimum cell-phosphorus content of plankton algae is a light- and temperature-independent species constant was investigated using the marine plankton diatom Thalassiosira rotula. To what extent the maximum cell-phosphorus content is also a constant, light- and temperature-independent quantity has been tested in parallel. At 2.5°C and 3.03 nE cm-2 s-1 the minimum and maximum cell-phosphorus contents are greater than the values for 16°C and 8.93 nE cm-2 s-1 by a factor of 5.7. The light intensities were kept near the light saturation for the growth rate for all experimental temperatures (2.5, 6, 12 and 16°C). The light dependence of the phosphorus content was tested at 12°C. For 1.43 nE cm-2 s-1 the minimum phosphorus content was lower by a factor of 2.5 than for 64.28 and 80.36 nE cm-2 s-1 respectively. The maximum P-content for 2.86 nE cm-2 s-1 was 3.9 times higher than for 64.28 nE cm-2 s-1 T. rotula is, on the basis of the stored P-content, only capable of between three and five cell divisions. The N/P atomic ratios were, depending upon light and temperature, between 56:1 and 226:1 for the minimum cell-phosphorus content, which implies a pronounced phosphorus deficiency. © 1986 IRL Press Limited.