Allozyme variation at 32 gene loci was studied in cave dwelling shrimps of the genus Troglocaris (fam. Atyidae). Populations were collected from several caves and wells belonging to different drainage systems in the Karst areas of NE Italy. Results suggest the existence of two sympatric, genetically distinct, biological species (provisionally indicated as species A and B in the T. anophthalmus complex) in the samples collected at the Duino Well, Sagrado and Comarie caves (Nei’s `D= 0.209). Samples from Gradisca and Aurisina are represented by only one (A) or the other (B) of the two species, respectively. Two remaining eastern populations from the Rosandra Valley also proved to be monospecific. However, they posses fixed alternative alleles markedly different from both species A and B suggesting that these two populations may belong to a third sibling species, C (A vs. C, `D = 0.346; B vs. C, `D =0.146). These data are discussed in relation to allopatric speciation of cave dwelling shrimp species, flooding periods of subterranean waters and the role of cave organisms as markers of this process.