The elemental composition of Ceratium hirundinella was determined in mixed phytoplankton samples collected over a 2-month period (late June to early September 1995). Electron probe X-ray microanalysis spectra of single cells routinely showed clear peaks of monovalent (Na, K) and divalent (Mg, Ca) cations, plus Si, P, S and Cl. Considerable variation in elemental concentrations occurred both within and between samples (20 cells). The overall ratio of monovalent to divalent cations was relatively constant at about 1.3. Intracellular concentrations of anions/cations and electronegative/electropositive elements were significantly correlated in most samples, suggesting a controlled internal balance of these ionic groups. Correlation analysis of elemental concentrations revealed clear patterns of statistical association within individual samples, with significant positive correlations between particular pairs of elements (Mg–P, K–Cl and K–P). Some pairs of highly correlated elements (particularly Mg–P) occurred in defined ratios throughout the samples, while others (e.g. K–Cl, Na–Mg) were variable. Factor analysis showed that elemental associations were determined by two Principal Factors in most samples, possibly related to insoluble and soluble components of cells. Decrease in the level of available P in lake water was associated with a sharp decrease in the intracellular P concentration and the concentration of other correlated elements. The estimated C/P ratio markedly differed from the classical (Redfield) value at times of low P availability. The concentration of P in cells of Ceratium was approximately 104 times greater than in the surrounding water medium (total P) compared with values of 102–103 for K.