We have isolated genomic clones for three loci encoding the phytochrome polypeptide of Zea mays, and have determined the entire sequence of one of them (phyA1) together with approximately 1 kb of 5' flanking DNA. The structure of this gene is highly conserved in comparison with other phytochrome-encoding genes (phy). The deduced amino acid (aa) sequence indicates that the maize phytochrome protein is 1130 aa long (125 kDa). Overall aa sequence identity is 88% with Avena and rice type A phytochromes and 65% with the type A phytochromes of the dicots, pea, zucchini and Arabidopsis. Northern analysis indicates that maize phy transcripts are down-regulated only two- to threefold in etiolated seedlings 3 h after a red light pulse, in contrast to Avena where a ten- to 20-fold decrease is observed. On the other hand, a more than tenfold reduction in maize phy mRNA abundance occurs in seedlings transferred to white light for 24 h. Several conserved sequence elements have been identified by comparison of the maize phyA1 and other monocot phy promoters, suggesting that these common regions may be regulatory elements involved in phy expression.