A hitherto-unrecorded stromatolite horizon in the Wolkberg Group of the Transvaal Sequence (1,950-2,300 million years) is described. The stromatolites are found in a thin sedimentary unit in a succession of basic lavas. The stromatolites and their enclosing sediments were probably formed in a volcanic lacustrine environment. It would appear that such environments were favorable for the development of the earliest stromatolite colonies, before their development on continental shelf-carbonate environments. The earliest shelf-carbonate deposits date back to some 2,000 million years ago and mark an "explosion" in the area of development of algal colonies. Assuming that the earliest algae were photosynthetic, this period must have marked the first period in the earth's history where significant volumes of oxygen were biologically contributed to the atmosphere.