The elemental composition of marine cyanobacteria is an important determinant of the ecological stoichiometry in low-latitude marine biomes. We analyzed the cellular carbon (C), nitrogen (N), and phosphorus (P) contents of Prochlorococcus (MED4) and Synechococcus (WH8103 and WH8012) under nutrient-replete and P-starved con- ditions. Under nutrient-replete conditions, C, N, and P quotas (femtogram cell 21 ) of the three strains were 46 6 4, 9.4 6 0.9, and 1.0 6 0.2 for MED4; 92 6 13, 20 6 3, and 1.8 6 0.1 for WH8012; and 213 6 7, 50 6 2, 3.3 6 0.5 for WH8103. In P-limited cultures, they were 61 6 2, 9.6 6 0.1, and 0.3 6 0.1 for MED4; 132 6 6, 21 6 2, and 0.5 6 0.2 for WH8012; and 244 6 21, 40 6 4, and 0.8 6 0.01 for WH8103. P limitation had no effect on the N cell quota of MED4 and WH8012 but reduced the N content of WH8103. The cellular C quota was consis- tently higher in P-limited than in nutrient-replete cultures. All three strains had higher C : P and N : P ratios than the Redfield ratio under both nutrient-replete and P-limited conditions. The C : N molar ratios ranged 5-5.7 in replete cultures and 7.1-7.5 in P-limited cultures; C : P ranged 121-165 in the replete cultures and 464-779 under P limitation; N : P ranged 21-33 in the replete cultures and 59-109 under P limitation. Our results suggest that Prochlorococcus and Synechococcus may have relatively low P requirements in the field, and thus the particulate organic matter they produce would differ from the Redfield ratio (106C : 16N : 1P) often assumed for the production of new particulate organic matter in the sea. The marine cyanobacterium Prochlorococcus is a small photosynthetic prokaryote (diameter, 0.5-0.8 mm) that is