Changes in the elemental composition of the blue-green alga Anabaena at Rostherne Mere (Cheshire, U.K.) were studied over an extended bloom period in 1996. Elemental analysis was carried out in relation to intracellular concentrations, intracellular elemental correlations, time series correlations and total quantities of elements in lake water. Most of the intracellular elements showed considerable variation both between and within samples. Correlations between these elements also varied considerably, though certain pairs – K/Cl, K/Mg, P/K and P/Mg were significantly correlated in at least 50% of the samples. Analysis of the time series demonstrated clear correlations between lake water P and intracellular P and Mg. The growth of Anabaena coincided with a considerable increase in the overall amount of chemicals contained in its population. The total amounts of chemicals in the lake water, however, always substantially exceeded those present in the population of Anabaena cells. Elemental relationships within cells are considered in relation to an elemental correlation pattern, consisting of three main groupings – Mg-K-P, Na-S and Ca-Cl. The structural model of intracellular elemental associations presented here may be helpful in the investigation of algal response to changes in environmental parameters.The incorporation of this model into a dynamic model simulating phytoplankton–environmental interactions is discussed.