Abstract Fifty different polyhydroxyalkanoic acid (PHA)-accumulating bacterial strains were investigated for the occurrence of phasin proteins bound to PHA granules and related to the GA24 protein of Alcaligenes eutrophus H16, by isolating PHA granules and Western blot analysis of granule-associated proteins employing antibodies raised against the GA24 protein. It could be demonstrated that the PHA granules of many poly(3-hydroxybutyrate)-accumulating bacteria exhibited ja similar protein pattern, and a predominant protein of 24 ± 2 kDa occurred in the granules of A. eutrophus strains A7, CH34, JMP222, N9A and TF93 exhibiting N-terminal amino acid sequences identical to that of the GA24 protein. Proteins bound to the granules of A. latus, Burkholderia caryophvli B. cepacia B. solanacearum, Pseudomonas glathei. Rhodobacter sphaeroides and Telluria mixta also gave positive immunoreactions. Granule-associated proteins of small size also; occurred in various strains of the Gram-positive bacteria Bacillus megaterium and R. ruher as well as in the Gram-negative bacteria Azotohacter sp., Chromatium vinosum, Comamonas acidovorans, Methylobacterium sp., Mycoplana ruhra, Paracoccus denitrificans, Pseudomonas sp., Rhodospirillum ruhrum, Rubrivivax gelatinosus and Thiocystis violacea; however, they gave no immunoreaction. This study clearly demonstrated that phasins are wide-spread if not essential in PHA-accümulating bacteria.