The seasonality of sexual reproduction was studied in three photosymbiotic Trididemnum species, T. miniatum, T. clinides and T. nubilum, inhabiting the same site. In all species, the colonies lacked gonads and/or embryos during the cold season, while gonads were found in most colonies for several months in the warm season. There were some interspecific differences in the duration of the breeding season. Although water temperature is thought to be the principal factor confining sexual reproduction in these photosymbiotic ascidians, other factors, e.g. precipitation, potentially affect fecundity.