Mitochondrial DNA restriction fragment patterns have been used to investigate the Chorthippus biguttulus group: a group of morphologically similar grasshopper species with very distinct male calling songs. Ten haplotypes were identified in 21 populations, representing six species. Haplotype diversity was as great within species, and in some case within populations, as it was among species and no clear geographical patterns of haplotype distribution were revealed. In the light of grasshopper range changes since the last glaciation it is unlikely that these data can be explained entirely by the retention of ancestral polymorphism. This suggests that hybridization has occurred during periods of grasshopper range expansion since the last glacial maximum, allowing mtDNA transfer between populations that have previously diverged in allopatry.