Four marine dinoflagellates, Amphidinium carterae Hulburt, Ceratium tripos (O.F. Mull.) Nitzsch, Prorocentrum minimum (Pav.) J. Schiller and Scrippsiella trochoidea (Stein) Loeblich III were grown as dilution cultures at 18.degree. C, S [salinity] = 29.permill. and 30 .mu.E .cntdot. m-2 .cntdot. s-1 [E = einstein] at L:D = 14:10 h. In nutrient-saturated cultures, the growth rates (double [doublings] .cntdot. day-1) ranged from 0.38 for Scrippsiella to 0.80 for Prorocentrum, and C content (pg .cntdot. cell-1) from 83 for Amphidinium to 6900 for Ceratium. The atomic N/C ratio ranged from 12-13 for Ceratium and Scrippsiella to 15-17 for Prorocentrum and Amphidinium. Under P-deficient conditions (growth rate 39-70% of the maximum), cellular P decreased considerably, but so did N, so that the N/P ratio was only slightly affected. There was a concomitant increase in C content per cell of 1.2- to 1.7-fold. Alkaline phosphatase activity was virtually nil in nutrient-saturated cells, but was readily demonstrable in all species when P-deficient.