Steady-state transcript levels of nuclear (rbcS, cab) and plastid (rbcL, psbA) encoded photosynthesis-specific genes were determined at noon and 05.00 h in different developmental stages of tomato fruits (7-35 days after anthesis). Small alterations are observed in mRNA levels for the small subunit (ssu) and large subunit (lsu) of RuBPC/Oase and the QB-binding protein of photosystem II at these two time-points, while significant steady-state transcript level fluctuations are detectable for the light harvesting complex protein. LHCP II transcripts accumulate during the day, and decline to low levels during the night. In contrast, the LHC II protein levels remain constant during the same period of development. A detailed analysis of transcript levels of the nuclear and plastid genes at 4-h intervals throughout a 38-h period demonstrates that LHCP II mRNA accumulation is highest at noon and lowest at 04.00 h. The transcripts of the ssu and lsu of RuBPC/Oase, photosystem I and II reaction center proteins, as well as the beta-subunit of the mitochondrial ATPase and the beta-subunit of tubulin, accumulate during the night and decrease to low levels in the afternoon. The transcript levels of the genes examined in this study fluctuate with certain periodicities. We suggest that gene expression in developing tomato fruits is at least partially controlled by diurnal rhythms, which are therefore also operational in other organs, besides leaves, of higher plants.