Genetic load in Tisbe (Copepoda, Harpacticoida). II. T. holothuriae and T. bulbisetosa from the lagoon of Venice. Two populations of Tisbe holothuriae coming from two lagoon localities different as to their environmental heterogeneity but similar in their environmental variability, have been examined. The first station is located near Alberoni at a distance of about 3 kilometers from the sea; the second one is toward the inner part, near Malamocco, at about 6 kilometers from the open sea. The benthic community at the Alberoni locality is richer than at Malamocco. Several statistics differ in the two populations: the mean number of eggs per female, the minimum developmental time, the mortality and sterility increment due to inbreeding. The last two measures permit an estimate of the “concealed genetic variability”. The genetic load, expressed as the number of lethal equivalents and sterility equivalents, is higher in the Alberoni population: insofar as the genetic load may provide an estimate of the genetic variability, these results suggest the hypothesis that the Malamocco population, more marginal and living in a poorer community, possesses a lower genetic variability. The genetic loads found for T. bulbisetosa and T. holothuriae from Malamocco are very close to each other. It is suggested that the higher genetic variability shown by the Alberoni population is maintained by environmental heterogeneity, according to the niche—with gene—variation hypothesis.