Four monogenic recessive tomato (Lycopersicon esculentum Mill.) mutants at the temporarily red light-insensitive (tri) locus (tri 1, tri 2 in the genetic background breeding line GT; tri 3, tri 4 in the genetic background cultivar Moneymaker) were studied. These mutants had slightly longer hypocotyls under white light than the wild type (WT). Western-blot analysis showed that the tri 1 mutant was deficient in a relatively lightstable phytochrome apoprotein (116 kDa) that was recognized in the WT by an antibody to tobacco phytochrome B; tri 2 had a 166-kDa band reduced in abundance; and tri2 and tri 4 had bands reduced in molecular mass, approx. 105 and 95 kDa, respectively. These patterns were also found in light-grown plants. Northern-blot analysis for PHYB1 mRNA showed for tri e a transcript approx. 2 kb larger, for tri 4, a transcript of WT size, but much reduced in abundance and for tri I and tri 3 transcripts equivalent in size and abundance to WT. In these mutants the transcripts of other members of the tomato phytochrome gene family (PHYA, PHYB2, PHYE, PHYF) were indistinguishable in size and abundance from WT. Thus, it appears that the tri locus specifically affects PHYB1 gene expression. Unlike phytochrome-B mutants in other plants, de-etiolated seedlings of the tri mutants exhibited normal responses to end-of-day far-red (EODFR) light and supplementary far-red light during the day. Since the holophytochromes of types B 1 and B2 (phyB 1 and phyB2) are closely related, it is proposed that there might be redundancy between them for these responses.