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Molecular chaperones of the chaperonin family occur in prokaryotes and in plastids and mitochondria. Prokaryotic and mitochondrial chaperonin-60 oligomers (Cpn-60) are composed of a single subunit type (p60cpn-60). In contrast, preparations of purified plastid Cpn-60 contain approximately equal quantities of two polypeptides, p60cpn-60 alpha and p60cpn-60 beta, with slightly different electrophoretic mobilities. We have isolated cDNA clones encoding plastid p60cpn-60 alpha and p60cpn-60 beta polypeptides from Brassica napus and Arabidopsis thaliana. The unexpected degree of sequence divergence observed between p60cpn-60 alpha and p60cpn-60 beta raises questions concerning the structure of the oligomer and the functions of these polypeptides. We have also found an amino acid sequence motif within all p60cpn-60 sequences which resembles the p10cpn-10 sequences.