Laguna Guerrero Negro and Ojo de Liebre are large, restricted embayments characterized by an arid climate and by a salinity ranging from that of normal seawater to the hypersalinity of sabkha environments. Intertidal microbial mats develop in some of the slightly hypersaline marshes and in many of the moderately hypersaline flats. Each major mat type corresponds to the lower, middle, or upper intertidal. In the lower intertidal mat, the photosynthetic horizon is a bilayer composed of: 1) a surface, blue‐green algal layer (about 3‐mm thick) dominated byMicrocoleus chthonoplastes;and 2) an underlying purple layer (about 2‐mm thick) of photosynthetic bacteria, predominantlyChromatiumsp. The mat may accrete to a thickness of 10 cm or more. It is characterized by fine laminations of alternating layers of blue‐green algae and of photosynthetic bacteria. The middle intertidal mat, dominated byLyngbya aestuarii,is relatively thin (usually 1 or 2 cm). Only a very thin, lower horizon of purple photosynthetic bacteria develops in this mat. One‐cm tall pinnacles that resemble the Precambrian stromatolite, “Conophyton,”; occur locally in theLyngbyamats. The upper intertidal is characterized by a crusty, wrinkled mat dominated byCalothrix crustacea;the total thickness of these mats is about 0.5 cm. None of the mats is lithified.