Described here are the first quantitative measurements of absolute amounts of mRNAs transcribed from individual members of a phytochrome gene (PHY) family. The abundances of PHY mRNAs were determined for dry seed and for selected organs of green-house-grown tomato (Solanum lycopersicum L.) seedlings and mature plants. With a Phosphoimager, absolute amounts of PHYA, PHYB1, PHYB2, PHYE and PHYF transcripts were measured with reference to standard curves prepared from mRNA fragments synthesized in vivo. Methodology was developed permitting the use of polymerase chain reaction (PCR)-generated probes derived from a highly conserved region of PHY, obviating the necessity to clone cDNAs and to isolate probes derived from their 3' non-coding regions. In dry seeds, PHYB1 mRNA appeared to be most abundant (4-5 mumol/mol mRNA) while in all other instances PHYA mRNA predominated. In seedlings, PHYB1, PHYB2, PHYE, and PHYF mRNAs were most abundant in the shoot (25-87 mumol/mol mRNA) while PHYA mRNA was most abundant in the root (325 mumol/mol mRNA). In adult plants, the levels of PHYA. PHYB1 and PHYE mRNAs were relatively uniform among different organs (approx. 100, 75, and 10 mumol/mol mRNA, respectively). In contrast, PHYB2 and PHYF were expressed preferentially in ripening fruits (35 and 47 mumol/mol mRNA, respectively), indicative of a possible role in fruit ripening for the phytochromes they encode. In general, the order of decreasing abundance of the five mRNAs for both seedlings and mature plants was PHYA, PHYB1, PHYE, PHYB2 and PHYF. Based upon observations that relatively modest changes in the extent of PHY expression result in changes in phenotype, the differential expression of each of the five tomato PHY described here is predicted to impact upon the spatial expression of biological activity of each phytochrome.