The polyhydroxyalkanoic acid (PHA) synthase gene (phaCAc) of a species of Acinetobacter isolated from an activated sludge treatment plant was cloned by heterologous complementation in a poly-beta-hydroxybutyrate (PHB) negative mutant of Alcaligenes eutrophus. Nucleotide sequence analysis of phaCAc revealed an open reading frame of 1770 bp with potential to encode a 67.7 kDa protein. The deduced amino acid sequence displays high similarity to other PHA synthase proteins. Probing with an internal region of phaCAc revealed that the PHA synthase gene may be present in more than one copy and may occur at both plasmid and chromosomal locations in Acinetobacter spp. This is the first organism for which evidence has been presented to suggest that a gene involved in PHA metabolism is plasmid-encoded. Purification of PHB granules from sucrose gradients identified proteins of 38 kDa, 41 kDa and 64 kDa which may have a role in PHB metabolism.