The elemental composition of Ceratium hirundinella cells was investigated in mixed phytoplankton samples collected from the water column of a stratified lake. X-ray rnicroanalysis (XRMA) routinely detected Mg, Si, P, Cl, K and Ca with occasional peaks of Na. Al and Fe. Cell concentrations of most elements showed no significant variation within the epilimnion, but Mg, P, C1 and K were significantly lower (and Si higher) in cells from the metalirnnion. Ratios of Mg/K, P/K, Mg/P, monovalent/divalent cations and diffusible anions/cations were constant throughout the sampled water column. Within depth populations of Ceratium, concentrations of each element varied considerably, approximating (except Si) to a normal distribution. Approximately 5 % of all cells had abnormally low concentrations of K and were possibly undergoing senescence. Correlation and factor analysis demonstrated a major statistical association between Mg, P, S and K in Ceratium cells throughout the depth samples. The XRMA results suggest an underlying homogeneity in the population of Ceratium throughout the sampled water column, consistent with the known high mobility and rapid migration of these cells within the water body.