Oceanographers now recognize two distinct classes of particles in seawater, broadly categorized as suspended and sinking. The former class dominates the standing stock of particulate matter in the ocean and the latter class dominates the exchange between the surface waters and greater ocean depths1. The downward vertical flux of particulate organic matter (POM) in the open ocean exhibits a non-linear decrease with increasing water depth2-6, and greater than 75% of the net POM loss occurs in the upper 500 m of the water column6. Because sinking particles contain viable, metabolically active microorganisms7-12, the process of microbial decomposition is considered to be an important mechanism controlling POM flux. This model is consistent with the observed correspondence between POM flux and dissolved inorganic carbon concentrations13, and with the reported selective loss of biochemically labile compounds from sinking particles14-17. From our experiments, however, we conclude that the large sinking particles are, in general, poor habitats for bacterial growth and therefore unlikely sites for the active remineralization of organic matter. Our results require a shift in the emphasis of current ideas of particle decomposition from microbes attached to rapidly sinking particles to the microbial populations which are either free-living in the water column or attached to suspended (non-sinking) particulate matter.