The growth response of the PSP‐producing dinoflagellateAlexandrium minutum, cultured from a shellfish poisoning outbreak in 1993 in the Bay of Plenty, was studied in semi‐continuous cultures, using 54 combinations of irradiance and different nitrogen (N) sources (nitrate (NO3−), ammonium (NH+4), urea) and concentrations. Irradiance had more influence on growthof A. minutumcultures enriched with NO3−than on cultures enriched with either NH4 or urea. At the high irradiance level (100 μmol m−2s−1), the greatest growth (0.5 doubling d−1) was recorded in cultures enriched with NO3−, followed by cultures enriched with NH4+and urea, respectively. For cultures enriched with either NH4+or urea, nutrient concentrations contributed to greater variations in growth than irradiance. At high irradiance, growth ofA. minutumpeaked in cultures enriched with about 25μMof either NH4+‐N or urea‐N. Growthof A. minutumwas substantially reduced when grown at 50 μM of either form of “recycled” N, and growth ceased at concentrations ≥ 100 and ≥ 200 μMof NH4+‐N and urea‐N, respectively. Growth ofA. minutumappeared to saturate at relatively low irradiance (Ik< 40 μmol m−2s−1). These results imply thatA. minutumis able to substain reasonably good growth rates, even in poorly illuminated depths within the water column.