The manganese ores of Groote Eylandt in the Gulf of Carpentaria, Australia, occur in a series of sandy claystones of Lower Cretaceous (Albian) age which overlie unconformably the middle Proterozoic sandstone basement. The ore minerals, chiefly pyrolusite and cryptomelane together with minor manganite, romanechite, braunite, lithiophorite, nsutite, todorokite, birnessite, and chalcophanite, occur as an almost continuous stratum of uncemented pisolites; cemented pisolites; dense textureless bands, pebbles, and boulders; and concretionary masses. The ores contain fossil evidence of microbiological activity, including algal stromatolites, oncolites, coccoid microfossils, and other microfossils enclosed in metacolloidal manganese oxides. Mineralogical investigations show that there was a close association between microbial activity and manganese oxide precipitation. The various theories relating to the origin of the Groote Eylandt manganese ores are discussed and a petrogenetic sequence involving biogeochemical precipitation of manganese oxide gels, sedimentary formation of pisolites and concretionary strata, diagenesis, and lateritization is outlined.