POLY-β-HYDROXYBUTYRIC acid has been found to occur very infrequently in Pseudomonas although it is very common in the related genera Chromobacterium, Spirillum and Vibrio 1,2. It is absent in authentic strains of P. aeruginosa, P. fluorescens, P. aureofaciens, P. convexa, P. schuylkilliensis, P. pavonacea, P. lemonnieri, P. mildenbergii, P. syncyanea and P. chlororaphis. Of classified strains tested it only appears to be present in the animal and the plant pathogens, P. pseudomallei 3 and P. solanacearum, which are scarcely typical of the genus. It has also been found in P. antimycetica 4 and a group of related organisms, which appear to be pseudomonads, referred to in previous publications1,2 as “pseudomonads related to Hankey Culture Collection (HCC) B175”. This group has now been studied in some detail and should, we feel, be considered members of Pseudomonas.