PURPOSE: To oxidatively deposit Mn existent in a liquid as MnO2 for its recovery, by cultivating bacteria having the power of oxidizing Mn in an Mn- contg. culture medium, and bringing the obtd. cultured biomass into contact with an Mn-contg. liquid in the absence of a nutritive source.

CONSTITUTION: In recoverying Mn from Mn-contg. water, e.g. waste water from an Mn mine, bacteria having the power of oxidizing Mn belonging to genus Pseudomonas are cultivated in an Mn-contg. culture medium, and the obtd. cultured biomass is repeatedly brought into contact with the Mn-contg. liquid under a condition substantially free from a nutritive source. Dissolved Mn in the liquid is fermentatively oxidized on the surface of the biomass and deposited as dark brown MnO2 onto the biomass. After said biomass is settled down, a supernatant liquid is removed to collect the biomass. Hence, MnO2 is recovered.