Bacterial genes responsible for poly(3-hydroxybutyrate) (PHB) biosynthesis were targeted to plant peroxisomes by adding a carboxy-terminal targeting sequence. The enzymes evidently were transported into peroxisomes, retained their catalytic activity, and reacted with peroxisomally available precursors because PHB synthesis in transgenic plant cells was localized to peroxisomes. Up to 2 mg/g fresh weight PHB was produced in suspension cultures of Black Mexican Sweet maize cells after biolistic transformation with three peroxisomally targeted bacterial genes. An equilibrium effect is proposed to explain the unexpected existence of (R)-3-hydroxybutyryl-CoA in plant peroxisomes.