Previous genetic analysis identified a component, RED1, that is required for normal de-etiolation of Arabidopsis thaliana (L.) Heynh. seedlings in continuous red light (Rc). red1 mutant seedlings exhibit elongated hypocotyls and reduced cotyledon size specifically in Rc and not in continuous far-red light (FRc). Here, we show that red1 is allelic to sur2 and atr4, and is defective in the cytochrome P450 CYP83B1, an enzyme required for normal auxin homeostasis. Two alleles of atr4, like red1, exhibit increased hypocotyl elongation and reduced cotyledon expansion in Rc but not in FRc. We further show that CYP83B1 transcript levels are elevated specifically in Rc-grown seedlings when compared with seedlings grown in darkness or FRc. Hence, the Rc-specific phenotype of the red1 mutant may indicate that Rc-induction of the CYP83B1 transcript is necessary for normal seedling de-etiolation in the wild type.