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Parthenogenetic insects are in general polyploid. These polyploid parthenogenetic forms seem to be superior competitors compared to their diploid and sexual relatives. Numerous studies have shown a surprisingly high clonal diversity in parthenogeinetic insects. We have studied Otiorhynchus scaiber, a common flightless weevil. O. scaber consists of three forms: one diploid sexually reproducing form with a very limited distribution and two widely distributed polyploid parthenogenetic forms. All forms, diploids and polyploids, coexist within the central area of distribution where both genetic and clonal diversity are highest. Diversity and degree of ploidy decrease towards the margin of distribution, so that only tetraploids inhabit the margins. This geographical pattern of distribution is a common phenomenon in parthenogenetic animals with different ploidy levels. We suggest that the high clonal diversity in populations of O. scaber is due to a continuous transition from diploid sexuality to triploid and finally to tetraploid parthenogenesis. We propose that the transition is caused by chance fertilisations of unreduced eggs.