Abstract.Two distinctive patterns of microbial destruction of mineral shell in mussels from the Florida Escarpment (3266 m water depth) are described. One pattern follows a surfacial selective removal of intercrystalline organic matrix creating caries‐like pits. Pitting of the shell resembles spreading bacterial colonies and results in substantial thinning of the original shell. The second pattern consists of an extensive system of sacs with branched and tapered extensions penetrating deep into the shell. Both activities are carried out by unknown microbial agents on live mussels, as evidenced by the presence of repair nacre, which often underlies effected areas. Shell destruction is primarily biological, providing conditions for enhanced chemical dissolution of the mineral shell.SummaryDamage and removal of the protective periostracum layer of deep‐sea mussels by various eukaryotic and prokaryotic microorganisms (Hook&Golubic, 1988, 1990, 1992) exposes the mineral portion of the shell to microbial destruction. This biogenic destruction exceeds the rates of inorganic carbonate dissolution in the area of study. Two types of destructive agents attack the shells of live mussels. The first type forms shallow caries along the interface between periostracum and the mineral shell, primarily attacking intercrystalline organic matrix. The second type represents boring microorganisms, morphologically similar to chytrids, that penetrate and permeate the entire mineral portion of the shell. These activities significantly weaken the shell structure and increase its internal poros