The species Artemia salina (L.) is composed of a group of differentiated units, which cannot interbreed because of differences in the manner of reproduction (Artom, 1931; Barigozzi, 1944, 1946, 1957). Artemia salina has, therefore, been termed by Artom "specie collettiva" also taking into consideration the different ploidy degrees. In fact, parthenogenetic females (belonging to indefinitely parthenogenetic stocks) produce only parthenogenetic females if mated with males, even though they have the same number of chromosomes (42 in somatic cells, 21 bivalents in meiosis). Some instances of reproductive isolation within the bisexual group have been described more recently. Kuenen (1939) pointed out that shrimps from California cannot be crossed with bisexual animals from Sardinia. A small morphological difference (presence or absence of abdominal hairs) was found between European and American Artemia and on this ground it has been thought that the California individuals belong to a different species: Artemia gracilis Verril, 1869. According to Lochhead (1941), the morphological difference is not confirmed. The second case, discovered by Barigozzi and Tosi (1959), refers to crosses between Californian and Great Salt Lake (Utah) Artemia, both bisexual. The chromosome number of individuals used for crosses proved to be the diploid one (42) for the Californian Artemia and the nearly tetraploid one for those from the Great Salt Lake. However, as the counts on Artemia of the Great Salt Lake indicate, the situation is very complicated and requires re-examination using a more suitable technique. Nothing is known about morphological differences between the two populations. The third case of reproductive isolation was reported by Gilchrist (1960) who failed to obtain successful crosses between North African and Californian Artemia. Both samples were considered by the author as diploid, although Barigozzi (1944) demonstrated that the California populations can be chromosomally heterogeneous, i.e. heteroploid. Gilchrist observed that between the North African and the American individuals there is a difference in the furca, which is "shorter" in the former. No cytological check has been made. The fourth example has been reported by Bowen (1965), who found reproductive isolation between American and San Bartolomeo (Cagliari, Sardinia) Artemia, all considered as bisexual diploid. More precisely, Bowen's findings refer to crosses between San Bartolomeo wild type and w/w stock #12 or w/w stock #9 individuals. Both mutant stocks derived from American populations (the former from Quemado, New Mexico, the latter from a cross between Utah and Californian individuals) and gave fertile crosses if mated with specimens from several other American populations. The author did not make any cytological check, but, on the other hand, discovered some morphological differences between American and San Bartolomeo males and females. From these data it can be concluded that, within the Artemia salina species, "races"y or units can be distinguished, which are slightly differentiated morphologically, but reproductively fully isolated. This leads to the postulation of genotypic 1 These studies were aided by Contract NR 104-862, between the Office of Naval Research, Department of the Navy, and the Istituto di Genetica (University of Milan).