The rbcS1, 2 and 3 genes of Phaseolus vulgaris are identical in coding sequence and we have studied their expression using gene-specific probes derived from their 3' non-coding regions. The genes differ in their relative levels of expression but show only minor qualitative differences in their regulation. Transcripts of the three genes are undetectable in primary leaves in the imbibed seed, accumulate early in leaf expansion reaching a maximum 7-10 d after sowing and decrease to low levels by the time expansion is complete. Both dark-grown and light-grown primary leaves exhibit this ontogenetic pattern of expression, although the light-grown leaves have two to three times more rbcS transcripts. Light can over-ride the ontogenetic control of rbcS expression; for example, when 7-d-old dark-grown primary leaves are illuminated there is a 6- to 12-fold increase in the transcript levels of the rbcS genes. Transfer of illuminated leaves to darkness results in the loss of transcripts of all three genes, but rbcS2 transcripts persist in the dark-adapted leaves. Possible physiological mechanisms of the ontogenetic regulation of expression are discussed.