According to the results of screening acidification activity of 24 species of micromycetes in vitro, 18 of them produced organic acids in artificial medium. The fungi from genera Penicillium and strains of Aspergillus niger have the most active production of organic acids. The ratio of acids formed by fungi on a liquid glucose-containing medium changes during ontogenesis: gluconic acid and acids of the Krebs cycle dominated in a log phase of growth; oxalic acid accumulated in a stationary phase while concentrations of other organic acids reduced. Oxalic acid is the main acid produced by fungi in a culture and apparently in a natural habitat. Formation of gluconate improves the efficiency of other carbon substrates consumption by fungi. Citric, succinic, malic, and fumaric acids are produced only at higher concentrations of carbohydrate and high ratio of C/N. Production of oxalic acid by micromycetes increases on the media containing only nitrate nitrogen. Zn in concentration up to 2 mM induces secretion of oxalic acid on nitrate media. Copper has stimulation effect on oxalic acid production only in very low concentration (25 µM). Presence of Zn in culture leads to formation of oxalate Zn crystals on the surface of mycelium. On ammonium-containing culture Zn and Cu does not stimulate oxalic acid production, but induce significant morphological changes.