The Halloween genes of Drosophila melanogaster were first described in the 1980s using cytogenetic methodology. During the past several years the genes have been cloned, expressed and the gene products have been characterized as cytochrome P450 enzymes (CYPs) and four have been functionalized as mediating the final steps in the biosynthesis of the arthropod molting hormone, 20-hydroxyecdysone (20E). A fifth has now been studied in detail and shown to be required for ecdysteroidogenesis but its exact function has yet to be elucidated. Since both insects and crustaceans utilize 20E as their principal molting hormone we have examined by BLAST search the genome of Daphnia and demonstrated the existence of Halloween gene orthologs in this crustacean, indicating that these genes play an identical role in this class as they do in insects. Further examination of the data bases representing Lepidoptera, Coleoptera, Hymenoptera and other Diptera allowed the development of a phylogenetic scheme for this gene family and suggests that the Halloween genes and vertebrate steroidogenic P450s originated from common ancestors that were perhaps destined for steroidogenesis, and arose before the deuterostome-arthropod split.