DURING an examination of the metabolic products of an unidentified purple-pigmented pseudomonad, a fraction, soluble in chloroform and insoluble in ether, was found to comprise 30 per cent of the dry cell weight. The material was isolated as a colourless solid, m.p. 168° C., which gave on alkaline hydrolysis a 70 per cent yield of crotonic acid. (Identified by m.p., mixed m.p. and preparation of the anilide and p-bromphenacyl ester.) The crotonic acid could also be obtained in 60–70 per cent yield by destructive distillation of the original solid. It was thought likely that the solid was a β-hydroxybutyric acid polymeric ester and this was confirmed by infra-red spectroscopy (ref. 1 and Farmer, V. C., personal communication) and by analysis (found, percent; C,55.1; H, 7.1. Calculated for (C4H6O2)n: C, 55.8; H, 6.8). Similar polymers have been described from Bacillus megaterium 2, Azotobacter chroococcum 3 and Micrococcus halodenitrificans 4.