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The gastropod genus Caldicochlea is endemic to the Dalhousie Springs in the arid north of South Australia. The genus contains two species, or species complexes, Caldicochlea harrisi and C. globasa that overlap morphologically but can be distinguished genetically, principally by allozymes at the Lap, Sordh-1 and Sordh-2 loci. C. harrisi is found in many of the approximately sixty active springs in the 70 km(2) of the Dalhousie complex, but C. globosa is confined to the northern half where there are larger, hotter springs. The species co-occur in many springs in this area, generally with apparently none, or low levels of hybridization, as judged by the absence of heterozygotes at the Lap and Sardh-2 loci. Introgression is apparently low and bidirectional. In two springs within the current ranges of both species, Lap genotypes occur in Hardy-Weinberg expected frequencies. Genotypes at other loci suggest that this is unlikely to be due to the independent evolution of Lap polymorphisms in these springs. Reproductive isolation may have broken down or may not have been established in these springs. For the latter to be so, reproductive isolation must have evolved through intra-spring interactions between the species, and independently in different springs. Isolation would not be fully established, if at all, until after secondary contact of their colonizing populations. (C) 2000 The Linnean Society of London.