Abstract: We investigated mitochondrial Cytochrome b sequences from all 15 members of the enigmatic Paramecium aurelia species complex (Ciliophora). The analysis revealed high genetic distances between the different A aurelia species (6.1-19.8%) and a largely unresolved, star-like phylogenetic tree. This result strongly supports a rapid radiation in the evolutionary history of this species complex and it correlates well with the hypothesis that the extant species diversity may have originated from the neutral consequences of a whole genome duplication in the common ancestor of P. aurelia. (c) 2008 Elsevier Inc. All rights reserved.