This study examined the effects of oxygen concentration, pond temperature and irradiance on productivity and irradiance on productivity and CaCO3 formation of the coccolith-forming algae, Pluerochrysis carterae CCMP647 grown in semi-continuous culture in outdoor raceway ponds. During the day the oxygen content of the pond increases markedly and P. carterae photosynthesis is inhibited by these high O-2 concentrations with the inhibition increasing with increasing temperature. The high radiance outdoors presents less of a problem to photosynthesis and productivity as the algae can acclimate well to high irradiances over a period of several weeks. Pond depth also effects productivity and this effect varies with season. During autumn, productivities were highest at depths of 13 to 16 cm, and decreased when the depth was increased. During summer productivity was much lower at 13 cm pond depth and increased when the depth was increased to 16, 18 and 21 cm. Heating the ponds in the morning by approximately 3 to 5 degrees C improves productivity by 11%-21%, presumably because this allows the algae to photosynthesise faster in the conditions of low (O-2) which occur in the early morning.