A comparison is made between two Italian populations ofMacrothrix hirsuticornisfrom two basins with different characteristics: a lowland temporary pond and a mountain lake. A survey was carried out on the biological cycle which showed both in the wild and in the laboratory a high ecophysiological diversity. As far as the morphological characteristics are concerned, these populations have an accentuated variability with individuals displaying characteristics fromhirsuticorniss. str.,groenlandicaand intermediate forms. However there is the highest percentage ofgroenlandicaat high altitudes and ofhirsuticorniss. str. in lowland areas.