We have isolated several cDNA clones for phytochrome from a dicot, Cucurbita pepo L. cv. Black Beauty (zucchini), and have used them to study the regulation of Cucurbita phytochrome mRNA levels. A cDNA library was constructed from poly(A)(+) RNA isolated from etiolated Cucurbita hypocotyl hooks and enriched for phytochrome mRNA by size fractionation. This library was screened with a (32)P-labeled fragment isolated from an Avena phytochrome cDNA clone. Several putative phytochrome clones were isolated and mapped by restriction endonuclease analysis. On the basis of this analysis there is no evidence for the expression of multiple phytochrome genes in Cucurbita. Recent sequence analysis has confirmed that the largest of these clones, pFMD1 (∼3.6 kb), does indeed encode phytochrome and that it contains the entire amino acid coding sequence for Cucurbita phytochrome (33). RNA blot analysis has revealed that two polyadenylated phytochrome transcripts (∼5.6 kb and ∼4.2 kb) are present in both cotyledons and hypocotyl hooks of Cucurbita. In etiolated Cucurbita seedlings given a saturating pulse of red light, the abundance of both transcripts coordinately declines to 50-60% of the dark levels within 3 h and reaccumulates to dark levels within 24 h. Reversal of induction of this response by a far-red light pulse immediately following red light treatment is not observed, which is in contrast to the far-red reversibility of the red light promoted decrease in phytochrome mRNA abundance observed in Avena (6). Etiolated seedlings transferred to continuous white light also show a coordinate decrease in the levels of the two RNAs to ∼40% of the dark levels within 3 h. The magnitude of the light-induced decline in phytochrome mRNA abundance in Cucurbita is substantially less than the decrease previously reported for Avena (6).