The 17 beta-hydroxysteroid dehydrogenases (17 beta-HSDs) are key enzymes in the final steps of steroid hormone synthesis. 17beta-HSD type 1 (HSD17B1) catalyzes the reduction of estrone to estradiol, while type 3 (HSD17B3) performs the conversion of androstenedione to testosterone. Here we present a functional genomics study of putative candidates of these enzymes in the zebrafish. By an in silico screen of zebrafish EST databases we identified three candidate homologs for both HSD17B1 and HSD17B3. Phylogenetic analysis, unique expression patterns (RT-PCR) during embryogenesis and adulthood, as well as activity measurements revealed that one of the HSD17B1 candidates is the zebrafish homolog, while the other two are paralogous photoreceptor-associated retinol dehydrogenases. All three HSD17B3 candidate genes showed nearly identical, ubiquitous expressions in embryogenesis and adult tissues and were identified to be paralogs of HSD17B12 and a yet uncharacterized putative steroid dehydrogenase. Phylogenetic analysis shows that HSD17B3 and HSD17B12 are descendants from a common ancestor.