A cytogenetic study was performed on 25 species belonging to 5 tribes of Melolonthinae (Coleoptera: Scarabaeidae). The presumed Polyphagan ancestral male and female karyotypes 20,XY/20,XX, composed of 9 pairs of metacentric (M) or sub-metacentric (SM) autosomes, acrocentric (A) Xs and punctiform Y, were observed in at least one species from each tribe, indicating its preservation in the Melolonthinae common ancestor. The chromosome number conservation in most species indicates that chromosome fusions or fissions were infrequent. However, almost all karyotypes differ from each other by changes in chromosome morphology, which indicates that pericentric inversions are by far the chromosome changes which occurred most frequently. Inversions of M or SM into A form principally involved the small chromosomes, whereas inversions of M or SM into SM with a more distal centromere involved larger chromosomes. Thus, the general trend of chromosome evolution in Melolonthinae seems to be a progressive displacement of centromeres towards chromosome ends. Derived (apomorphic) acrocentric autosomes are clustered (26/33 occurred in 5/25 species) and differentiate congeneric species rather than genera or tribes. In spite of the small number of specimens by species generally studied, a heterozygote status for an inversion of M into SM or SM into A was detected in 2 species. It is concluded that pericentric inversions characterize the recent evolution of Melolonthinae, are involved in the speciation process and remain an on-going phenomenon. This potential role in the speciation of Melolonthidae, and possibly most Polyphagan beetles, has been totally overlooked up to now.