The yeast Abc1 protein acts as a chaperone-like protein essential for the proper conformation and efficient functioning of the respiratory complex III. By functional complementation of a yeast abc1 mutant, we have identified an Arabidopsis thaliana cDNA that corresponds to a single copy gene and encodes a protein sharing 45% similarity with the yeast Abc1p protein. Cytochrome spectra and respiratory activity measurements have shown that the plant protein allows a partial restoration of the complex III activity. No major difference in the steady-state level of ABC1At mRNA was observed in various plant tissues, suggesting that ABC1At is constitutively expressed in A. thaliana. Phylogenetic analysis revealed that the Abc1At protein belongs to a large family of proteins composed of two eukaryotic and one prokaryotic subgroups differing by their degree of similarity and probably by their function.