New cytogenetical data are provided for the three Ranunculus species occurring on Iles Kerguelen. Chromosome morphometrical analysis (idiograms) strongly argues for a relationship between them, with some indications of a more recent origin for R. moseleyi. Aneuploidy was found within the three species for the first time. Whilst only a slight aneuploidy exists in R. pseudotrullifolius and R. moseleyi, R. biternatus shows a tendency to hyperaneuploidy. Vegetative reproduction is likely to increase the number of plants showing such chromosome number irregularities. Aneuploidy could explain some of the differences between the species in ecological tolerance and distributional area.