Growth rates ofHeterosigma carterae(Hulburt) comb, nov., isolated from the 1989 bloom in Big Glory Bay, New Zealand, were studied in continuous batch cultures, with 48 combinations of light and nitrogen (N) sources: NO3–, NH4+, and urea. Light had more influence on cultures grown with NO3–than cultures grown with the other N sources. At 160 μmol photons m–2s–1, the growth rate ofH. carteraewas highest with NO3–, intermediate with NH+4, and lowest with urea. At lower irradiances, growth rates of NO3–‐grownH. carteraewere lower than those of either form of “recycled” N. For NH4+‐ and urea‐grown cultures, nutrient concentration contributed to a greater variation of growth than did light.H. carteraeexhibited signs of toxicity at high concentrations of NH4+and urea, particularly at low light levels. The implication of the variations in growth rate under different combinations of light and N sources is that maximum growth ofH. carteraein the field is probably attained in the well‐illuminated, upper water column with NO3–as the major N source. When either NH4+or urea is the major N source, a lower maximum growth rate is probably attained over a greater range of illumination deeper in the water column.