The production of microcystins (MC) from Microcystis aeruginosa UTEX 2388 was investigated in a P-limited continuous culture. MC (MC-LR, MC-RR, and MC-YR) from lyophilized M. aeruginosa were extracted with 5% acetic acid, purified by a Sep-Pak C(18) cartridge, and then analyzed by high-performance liquid chromatography with a UV detector and Nucleosil C(18) reverse-phase column. The specific growth rate (mu) of M. aeruginosa was within the range of 0.1 to 0.8/day and was a function of the cellular P content under a P limitation. The N/P atomic ratio of steady-state cells in a P-limited medium varied from 24 to 15 with an increasing mu. The MC-LR and MC-RR contents on a dry weight basis were highest at mu of 0.1/day at 339 and 774 microg g(-1), respectively, while MC-YR was not detected. The MC content of M. aeruginosa was higher at a lower mu, whereas the MC-producing rate was linearly proportional to mu. The C fixation rate at an ambient irradiance (160 microeinsteins m(-2) s(-1)) increased with mu. The ratios of the MC-producing rate to the C fixation rate were higher at a lower mu. Accordingly, the growth of M. aeruginosa was reduced under a P limitation due to a low C fixation rate, whereas the MC content was higher. Consequently, increases in the MC content per dry weight along with the production of the more toxic form, MC-LR, were observed under more P-limited conditions.