Androniscus dentiger is a terrestrial isopod distributed from Great Britain to North Africa, inhabiting humid edafic environments, superficial underground compartments and both natural and artificial caves. In this study allozyme data have been used to investigate the geographic variation and the genetic relationships of several populations of A. dentiger from Central Italy, using as outgroups populations from four congeneric species, A. calcivagus, A. cfr. subterraneus, A. spelaeorum, and A. degener. Multivariate analysis of A. dentiger allele frequencies indicates the existence of a group of populations (group A) distributed in a wide geographic area which are genetically slightly differentiated, and several populations (arbitrarily defined as group B) which show differentiation levels comparable to those observed between the morphologically well differentiated species. The low valley of the river Tiber seems to act as an effective geographic barrier between the populations from group A and the remaining ones. The genetic divergence between populations within the group A seems to have a recent origin. This is suggested by the low genetic distances and heterozygosity values within the group A, and by the very low number of private alleles occurring in this group. The high degree of intraspecific and interspecific genetic differentiation is not consistent with the levels of morphological differentiation traditionally used to distinguish different species within this genus. On the whole, these data suggest that A. dentiger might be considered as a complex of cryptic/sibling species.