Quantitative measurements of the absolute amounts of mRNAs transcribed from each of five phytochrome genes (PHYA, PHYB1, PHYB2, PHYE, PHYF) throughout the life cycle of a tomato plant and in response to changes in ambient light conditions are reported here. From their lowest level in unimbibed seed, all five transcripts increase by from 10- to 1000-fold during the first 24 h following the onset of imbibition, both in continuous darkness and in a greenhouse. In a greenhouse and on a whole-plant basis, all continue to increase throughout day 6, after which all but PHYE decline over the next week to a plateau at about one-half of the maximal value. PHYE mRNA differs in that it continues to increase in abundance during the first 2–3 weeks and thereafter remains at that maximal level. In adult plants, on a whole-plant basis and in decreasing order of abundance, PHYA, PHYB1, PHYE, PHYB2, and PHYF transcripts were present at approximately 120, 40, 40, 15, and 8 μmol μg–1 of poly(A)+-enriched RNA, respectively. The data are consistent with the demonstrated roles of phytochromes A and B1 during seedling development and lead to the suggestion that phytochrome E might have a more important role in mature plants. Somewhat unexpectedly, PHYA and PHYB2 expression patterns are very similar. In seedlings, PHYA and PHYB2 exhibit the greatest increase in expression following a light-to-dark transition, as well as the greatest decrease following a dark-to-light transition. PHYA and PHYB2 are also similar in that both exhibit comparable variation on a natural diurnal cycle, while PHYB1 also exhibits variation but with a markedly different phase. The diurnal variation in expression of PHYA, PHYB1 and PHYB2 is consistent with the possibility that one or more of the phytochromes they encode is important with respect not only to photoperiodic behavior but also to the regulation of other events whose photosensitivity varies during a diurnal cycle.