Maize seedlings, like seedlings of many other plants, are rich in cytochrome P450 (P450) enzyme activity. Four P450 genes (CYPzm1-4), isolated from a seedling-specific cDNA library, are characterized by a transient and seedling-specific expression pattern. The maximum steady state mRNA levels are reached at 3 days in root and at 7 days in shoot tissue, respectively. All four genes belong to one gene family and are closely related to the CYP71 family of plant P450 genes, which includes the enzymes of the ripening avocado fruit (CYP71A1) and eggplant hypocotyls (CYP71A2, A3, A4). The expression of these related P450 genes in monocot and dicot plants indicates that these enzymes play a significant role in plants; however, the in vivo enzyme functions are unknown. The divergence of the four members of the maize gene family is sufficiently high to account for different substrate and/or reaction specificity. Although the general expression pattern of the four genes is identical, the maximum steady-state mRNA levels vary in different maize lines. In situ hybridisation reveals the highest mRNA levels in the coleoptile, the first developed leaflets, the ground tissue of the nodular complex, and in the cortex and pith of the region of cell division in the root. The mapping of the maize CYPzm genes shows that, as in animals, P450 genes of the same family can be clustered. The presence of the CYPzm gene cluster in maize argues for generation of distinct plant P450 gene families by gene duplication.