Among vertebrates, true parthenogenesis (self-perpetuating all-female species) occurs only in reptiles; these species are of hybrid origin. To date, all diploid parthenogenetic reptiles examined exhibit some genetic diversity, resulting in the existence of more than one clone. The sole exception to this is the Caucasian rock lizard Lacerta rostombekovi, which appears to consist of only a single clone. © 1997 Elsevier Science Ltd. All rights reserved Introduction Although well-documented, the phenomenon of parthenogenesis is not well under-stood. Questions dealing with the ecology, genetics, frequency of hybridization, genealogical constraints on parthenogenesis, and mechanisms for failed meiosis are paramount. Among vertebrates, true parthenogenesis has been detected only in squamate reptiles, and especially among lizards (Vrijenhoek et al., 1989). All diploid parthenogenetic species examined have been shown to exhibit intraspecific genetic diversity; each 'species' consists of multiple clones, caused by either mutation, multiple origins or genetic recombination (Cole et al., 1 988; Parker, 1 979a; Parker and Selander, 1984). Clonal diversity has been related to distribution, ecological parameters, and inferred age of parthenogenetic species (Dessauer and Cole, 1989; Parker, 1979b). Because of their origin through hybridization, parthenogenetic species exhibit char-acteristics of F1 hybrids, including a high level of fixed heterozygosity. In hybridogenesis there must be a balance between too much genetic divergence between parental species, which would result in developmental failure, and hybridization, permitting viable offspring and backcrossing. This has been termed the 'balance hypothesis' (Moritz et al., 1989). The relationships among potential parental species require further study. The genus Lacerta contains several parthenogenetic species, all of which were formed by hybridization between bisexual species; in fact, parthenogenesis in reptiles was first discovered in this genus (Darevsky, 1958). One of these unisexual species, L. ros-tombekovi, arose from the hybridization of a male L. portschinskii and a female L. raddei (Darevsky et al., 1985; Moritz et al., 1992; Murphy et al., in press). The two parental tCorresponding author (ROSSM@ROM.ON.CA).