Abstract: A process for producing poly-3-hydroxyalkanoate (PHA) by culturing Alcaligenes on a low water-solubility aliphatic carboxylic acid and, or hydrolysable derivative of low solubility in pure water, by fermenting the organism in a growth step on a nutrient medium containing inter alia phosphorus in a quantity corresponding to the intended quantity of bacterial cells to be grown until cell growth stops or slows substantially, then in a PHA accumulation stage fermenting the grown cells by feeding said low-solubility carbon source while monitoring pH and adjusting it by addition of ammonia and/or alkaline alkali metal compound until a design quantity of PHA has been produced, and recovering PHA from the product. Further phosphorus may be fed to the accumulation stage at a rate sufficient to permit some bacterial growth but insufficient to permit growth to the exclusion of PHA accumulation.