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Laboratory studies were performed to determine the effect of temperature, salinity, seawater sources and culture media on the vegetative growth of clonal cultures of Gymnodinium catenatum isolated from Bahia Concepcion, Mexico. These isolates were heterothallic and isogamous. Exponential growth rates of G. catenatum in f/2 with different selenium concentrations and soil extract and GSe media were moderate. Maximum cell yields were obtained in GSe and f/2 media with selenium (10 � 8 and 10 � 7 M), while in f/2 medium with soil extract cell yields were considerably lower. The highest percentage of long chains was found in f/2 media supplied with selenium (10 � 8 M). The optimal temperature range for growth was 11.5-30 � C, with the highest growth rates between 21 and 29 � C. The range of salinity tolerated by G. catenatum changed with seawater source. With seawater from Vineyard Sound (Massachusetts, USA), G. catenatum grew at salinities from 15 to 36, with an optimal growth rate obtained at salinities between 26 and 30. With seawater from Bahia Concepcion, this species tolerated salinities from 25 to 40, with optimal growth at salinities between 28 and 38. Ecophysiological measurements reported here are consistent with the environ- ment of the bay, which has limited input of humic materials from runoff and high salinity and temperature. These data, when viewed with data from studies of globally distributed G. catenatum, demonstrate the ability of this species to live in a broad array of habitats.