Zygowilliopsis sp. WY7905 contained constitutive 17β-hydroxysteroid dehydrogenases. It effectively catalyzed the reduction of androst-4-ene-3,17-dione to testosterone. It also reduced other 17-oxosteroids to the corresponding 17β-hydroxysteroids. It was not active toward steroids with only C-3 or C-20 carbonyl groups.