Blue-green algae are a major agent of destruction of carbonate rocks and sediments, particularly in the littoral and shallow marine environments. The organisms discussed here are endolithic, that is, the individual algal filaments reside snugly in made-to-measure tunnels of their own, excavated chemically. Data presented here suggest that much of this geologically significant bioerosion is actually caused by what may be called digestion of the mineral substance, and is not, as previously thought, a more or less accessory effect of extracellular secretion. Endolithic cyanophytes may even turn out to be ‘rock-eaters’ in the true sense