Three forms of stromatolites mineralogically composed of hydromagnesite and aragonite occur in two ephemeral lakes near the Coorong Lagoon. None occur in the Lagoon or other lakes. The three forms are stratiform, globular and crenulate. Each is built by cyanophytes, the first two by monospecific mats (respectively coarse and fine morphotypes of Microcoleus chthonoplastes) and the third by a variable association dominated by Schizothrix cresswelli and Calothrix fasciculata. The shape of the stratiform stromatolites is due largely to desiccation and erosion although the flat mat surface seems to be characteristic of mats built by coarse M. chthonoplastes. The shape of the globular stromatolites results from the radial growth habit of the fine morphotype of M. chthonoplastes. The shape of the crenulate stromatolites results from a complex of environmental and biological factors. Environmental and biological factors interact to shape stromatolites and either can dominate. The evolution of stromatolite morphologies through the Precambrian can be due to biological evolution.