Marginal grain corrosion on calcareous grains present in the Lower Ordovician (El Paso Group) mound carbonates of west Texas, appears to be caused by the boring and perforating action of the primitive filamentous blue-green alga Girvanella . This boring and perforating action of algal filaments results in profound marginal corrosion of the skeletal and nonskeletal grains, and ultimately leads to grain breakdown and micrite formation within the corrosion rim. The mechanism of the algal boring and perforating process is unknown. Neither do we know much about the cause for the precipitation of micrite within the algal corrosion rim. Our observations tend to indicate that the micrite precipitation may be related to the algal life processes and decay. Partial recrystallization of the micritic corrosion rim or envelope around the grains occurs frequently. This may indicate prior leaching and the remobilization of the carbonate component during the penetrative stage of the algal filaments, or sometime during their decay. The boring and perforating action of the primitive filamentous algae appears to be a major influence in carbonate grain alteration, destruction, and eventual micrite formation. These primitive algae occur in sediments ranging in age from the Precambrian to the Recent where they are found abundantly in shallow water environments, generally shallower than 50 meters, and in all climatic zones. It is thought that the boring algae are important agents in early diagenetic particle and grain breakdown in carbonate sediments.