Morphometric analysis of cephalothorax length in successive growth stages of Calanoides acutus and Metridia gerlachei from three regions of the western Antarctic, i.e. Drake Passage, Bransfield Strait and Croker Passage (Antarctic Peninsula), indicated a diversity in size of these animals in these regions. The smallest specimens, on average, of both species (group A) were found at stations within range of the Bellingshausen Sea (warmer and less saline waters). The medium-sized specimens (group B) were present in areas influenced by the Weddell Sea (colder and more saline waters). The largest specimens (group C) were caught in Croker Passage, where the water column is isothermic, and temperature ranged from 0°C at the surface to -0.5°C at a depth of 1000 m. The differences in body size show that temperature appears to be the principal modifying factor and is particularly evident in the final growth stage of M. gerlachei (copepodit IV-V and adult - female) and in the initial such stage in C. acutus (copepodit I-III).