There is a paradox reported concerning observations of toxic blooms of Prymnesium parvum in winter in Texas (USA), and its growth performance in the laboratlry. Culture experiments indicate relatively high optimum temperature and salinity for growth, conditions different from those ofwinter in Texas. Because previous laboratory experiments were of short duration, it was suggested that long-term culturing of many generations could allow physiological and epigenetic changes, as well as natural selection to take place, leading to improved growth performance at low temperature and salinity. In this study, the growth and toxicity of P. parvum was investigated under relatively low temperature and salinity, for two phases of culture. The experimental results show, in general, that P. parvum grows more rapidly at higher temperature and higher salinity during both phases of long-term repeated culturing. When the two phases of culturing are compared, P. parvum grows more slowly at 11°C during Phase 2, while growth differs little at 20°C. Toxicity differs little between phases. These results show no evidence of acclimation or adaptation during culturing over the length of time examined. These results support earlier short-term experiments indicating that this Texas strain of P. parvum displays slower growth at temperatures < 20 °C and salinities < 2 ppt than it does at higher temperatures and salinities.