Changes in water chemistry and the major chemical and elemental constituents of Peridinium cinctum have been followed during the 1976 growth season (February-June) in Lake Kinneret. The decline of the bloom was not associated with changes in any single physical or chemical parameter of the lake water, although the ambient N:P ratio was higher before the peak of Peridinium. Carbon, nitrogen and chlorophyll in the cells were almost constant over the growth season, averaging 48, 4 and 0.9% dry weight, respectively. There was an initial drop in cell phosphorus from 0.7% to 0.2% dry weight which then remained stable after April. During the bloom period the carbohydrate and lipid content of Peridinium decreased, nucleic acid increased, whereas protein showed no definite trend. The results suggest that (a) Peridinium is not severely nutrient limited during most of the growth season; and (b) criteria based upon cellular composition used to define nutrient status in other groups of algae, may not be directly applicable to armoured dinoflagellates.