Abstract Colony growth, medusa bud development and hydranth degeneration were studied as functions of temperature (10, 15, 20, 25, 30 and 35°C) and salinity (15, 25, 35, 45 and 55%) combinations, in the athecate hydrozoan, Stylactis hooperi. The best growth rates were recorded over a three week period at 20°C in salinities of 25 and 35% for both male and female colonies. At 10 and 35°C, in all salinities, and in 15 and 55% at all temperatures, colonies showed little or no development. In low salinities male and female polyps developed at the same rate in both low and high temperatures, respectively. However, in high salinity, male polyps developed better at high temperatures while in female polyps, the contrary was observed. In general male colonies showed better development than female. Medusa buds developed only in a few temperature‐salinity combinations, the greater development being registered in female colonies at 30°C and 25%. At the extremes of the salinities and temperatures tested, 100% polyp...