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The level of transcripts of genes encoding the small subunit of ribulose 1,5-bisphosphate carboxylase/oxygenase (rbcS genes) in the primary leaves of Phaseolus vulgaris L. increases substantially when plants grown in a low fluence rate of white light (15 µmol m-2 s-1; 400–700 nm) are transferred to a tenfold higher fluence rate of identical spectral quality. To investigate which photoreceptor acts as the fluence-rate detector, plants grown for 16 d in low white light were transferred to blue-enriched or red light environments of various fluence rates. The results indicate that the fluence-rate-dependent increase in rbcS expression is mediated specifically by blue light. Red light of the same fluence rate, which was found to be equally effective in driving photosynthesis, had much less effect on expression, indicating that light absorbed by the photosynthetic pigments does not mediate the response. Moreover, there is no correlation of the transcript levels with either the cycling rate or photoequilibrium of the phytochrome system. Run-on assays with isolated nuclei indicate that blue light substantially increases the rate of rbcS transcription. Experiments with gene-specific probes show that individual members of the P. vulgaris rbcS gene family exhibit the fluence-rate-dependent, blue-light-mediated increase in expression.