The relationship between the specific cell division rate (mu(c)), the specific microcystin (mcyst) production rate (mu(mcyst)) and the cellular content of mcyst (Q(mcyst)) was investigated during growth of Microcystis aeruginosa strain CYA 228 cells in the field (microcosms), and the results were compared with previous data obtained from batch cultures. Growth of an easily recognizable unicellular culture alga in the field made it possible to evaluate different ways of expressing mcyst field data as the ratio of mcyst to dry weight, protein or chlorophyll a (Chl a) against the mcyst quota. The population of CYA 228 cells increased from day 1 to day 7, but decreased from day 7 to day 17. More than a threefold variation was observed in Q(mcyst) of M. aeruginosa cells under field conditions, which indicates that the relationship between mu(c) and mu(mcyst) was not strictly linear. The data from the present experiment support the hypothesis of a constitutive mcyst net production. The average Q(mcyst) of cells grown under field conditions was 85.7 +/- 43.6 ((x) over bar +/- s) fg mcyst cell(-1) compared to the average of 278 +/- 115 ((x) over bar +/- s) fg mcyst cell(-1) found previously under laboratory conditions. The less favourable light conditions during the field study probably caused the reduction in mcyst quota. The data highlight the importance of determining mcyst quotas or the ratio of mcyst to Chl a, because confusing interpretations arise from determining the gravimetric mcyst content or mcyst: protein ratios in the field.