Gephyrocapsa oceanicaandEmiliania huxleyiwere major components of extensive blooms in New Zealand's coastal waters from September to February, 1992/93. Unusually cold sea surface temperatures at that time were related to the El‐Nino phase of the Southern Oscillation and consequent climatic patterns.G. oceanicaco‐dominated withFibrocapsa japonica(Raphido‐phyceae) along the north‐east coast (spring‐early summer) in a bloom which immediately preceded a toxic dinoflagellate event.E. huxleyibloomed in the colder waters of Big Glory Bay, Stewart Island, where a significant water column temperature gradient and depleted nitrate and phosphate concentrations were recorded. An isolate ofE. huxleyifrom Big Glory Bay grew optimally at a salinity of 29 × 10−3, pH of 7.5–8.9, and temperature of 15–25°C. An isolate ofG. oceanicafrom Leigh grew optimally at salinity 17–29 × 10−3, pH 8.4–8.9, and 20–25°C.G. oceanicagrew with ammonium chloride, urea, or potassium nitrate as nitrogen source;E. huxleyigrew optimally with ammonium chloride. Maximum growth rates were 1.9 doublings d−1forE. huxleyiand 1.4 doublings d−1forG. oceanica.