The microbiological process for the production of hydroxy-heterocyclic carboxylic acid of formula ##STR1## wherein R.sub.1 means a hydrogen or a halogen atom and X means a nitrogen atom or a CR.sub.2 function, wherein R.sub.2 means a hydrogen or halogen atom, starting from the corresponding heterocyclic carboxylic acid. The process is performed so that an aerobic biomass which utilizes nicotinic acid, is cultivated in a molar ratio of nicotinic acid to mineral acid of 1 to 8. The ratio is assured over the entire cultivation phase. The the hydroxylation of the corresponding heterocyclic carboxylic acid of the general formula ##STR2## wherein R.sub.1 and X have the above-mentioned meanings, is performed with the biomass. Under these conditions, strain Pseudomonas acidovorans DSM 7205, strain Pseudomonas acidovorans DSM 7203, strain Alcaligenes faecalis DSM 7204 and strain Arthrobacter crystallopoietes DSM 7202 are concentrated in the cultivation phase.