Recent stromatolites produced by in situ aragonite precipitation occur in a hypersaline coastal pond on the west coast of Baja California, Mexico. These structures range up to 15 cm in height and consist of a centimeter-thick aragonite layer which encrusts substrate irregularities. The coccoid cyanophyte Entophysalis , commonly premineralized within the aragonitic crust, is the dominant biologic component of these stromatolites and probably caused precipitation of aragonite by photosynthetic removal of carbon dioxide and (or) bicarbonate. Lamination in the Entophysalis -deposited aragonite is irregular and poorly developed. The single colonial species of Entophysalis occurring in these structures exhibits a wide range of organization--from a few cells enclosed within a spherical sheath to hundreds of cells forming attached mammillate colonies. These colonies, occurring in the organic mat on the stromatolitic growth surface and permineralized within the aragonitic crust, exhibit selective degradation. Intracellular material degrades most rapidly and rarely is preserved within the aragonitic crust; the relatively thin inner sheaths which surround individual cells and small groups of cells within a larger colony are somewhat more resistant and occasionally are preserved within the crust; the tough outer sheath which envelopes an entire colony is most resistant and commonly is the only material preserved within the crust. These observations on Recent stromatolites and stromatolite-building microorganisms provide a basis for understanding some Proterozoic stromatolites and for interpreting their preserved eyanophytic microbiotas.