The Middle Precambrian problematical microorganismEosphaeraBarghoorn andEosphaera‐like structures known from Early and Middle Precambrian banded iron formations have been compared with the recently discovered Devonian volvocacean algaEovolvoxKaźmierczak and some modern colonial Volvocales. The volvocacean interpretation ofEosphaeraimplies that algal eukaryotes (green phytoflagellates) werc already prewmt in the earth biosphere before at least 1.9 b. y. ago (Gunflint Iron Formaticn), and probably before 2.7 b. y. ago (Soudan Iron Formation). The type of metabolism and thc oxygen requirements of modern colonial Volvocales indicate thatEosphaerawas most probably a photoorgano‐trophic (mixotrophic) organism able to live in the extremely oxygen‐deficicnt or anoxy‐genous Early Precambrian environment. As an oxygen‐releasing photosynthesizer,Eosphaeracould have played a considerable role in the production of free oxygen during the Precambrian. The abundance ofEospkaera‐like ferriferous structures in the iron microbands of many banded iron formations implies active participation of these organisns in the formation of Precambrian sedimentary iron ores. The exclusively fresh‐water habitat of extant volvocacean algae suggests that the Procambrian environments inhabited byEosphaeraw