The influence of organic and inorganic components of biofilms with microscopic fungi on the phase composition and morphology of precipitating calcium oxalates has been experimentally investigated. It is established that biofilm components may facilitate the formation of metastable calcium oxalates (dihydrous weddellite and trihydrous caoxite) in the crystallization range of monohydrous whewellite. It is shown that the decisive role in the formation of weddellite crystals in biofilms is played, along with oxalic acid, by the citric acid released by micromycetes, Fe3+ ions from the environment entrapped by the biofilm, and the ratio of Ca2+ ions and oxalate ions. The transformation of bipyramidal habit of weddellite into bipyramidalprismatic habit may be caused by the presence of Mg2+ and ions in the crystallization medium.