N. pungens multiseries (clone NPARL) was grown in nonaxenic batch culture under a range of growth conditions. Domoic acid (DA) was not detected during exponential growth, but prodn. promptly started at a rate of ∼1 pg DA/cell-day at the onset of the stationary phase, in this case induced by silicate limitation. Cellular DA reached a max. of 7 pg/cell; thereafter, DA prodn. continued at the same rate, with cellular levels remaining relatively const. due to concurrent release of DA into the culture medium. DA prodn. ceased in the absence of nitrogen during the stationary phase, but resumed when nitrate was added back to the medium. Low irradiance slowed the division rate of consequently delayed the attainment of the stationary phase, but DA prodn. rates were comparable with the control once stationary phase was reached. Cells during the dark period of a light-dark cycle, or placed into darkness, or in the presence of the photosynthetic inhibitor DCMU promptly ceased DA prodn. At least 3 conditions are required for DA prodn. by clone NPARL: cessation of cell division, availability of N during the stationary phase, and the presence of light. Growth in medium f/2 fulfills these requirements.