We report here that the catalytic β subunit of the mitochondrial ATPase/ATP synthase is encoded by a small multigenic family in the diploid tobacco Nicotiana sylvestris (nsatp2 genes). cDNAs and genes corresponding to the β1, β2 and β3 (pollen specific) isoforms previously detected by 2D-SDS PAGE were isolated. Nsatp2.1 and nsatp2.2 transcripts were found in all vegetative and reproductive tissues analysed. In contrast, nsatp2.3 transcripts were found exclusively in bicellular pollen. As a whole, steady-state transcript levels of nuclear nsatp2 and mitochondrial atp1 genes were found to be closely correlated.