The diploid oat species containing the Cp genome - Avena pilosa and A. clauda - were studied using C-banding, fluorescence in situ hybridization with probes pTa71 and pTa794, and electrophoresis of grain storage proteins (avenins). Species with the C genome differed considerably from the species of the A genome group in the karyotype structure, heterochromatin type and distribution, relative positions of the 45S and 5S rRNA gene loci, and avenin patterns. These facts confirmed that the C genome had diverged from the ancestral genome before the radiation of the various A genome. Presumably, further evolution of the A -and C genome species occurred separately.