The purpose of this study was to further our understanding of the role of particle-associated bacteria in phytoplankton degradation in the Chesapeake Bay, USA, and to identify environmental parameters that control production by free and particle-associated bacteria. Surface and bottom waters at 10 stations along the length of the Bay were sampled over a 2 yr period. Samples were analyzed for temperature, salinity, chlorophyll, phaeophytin, particulate protein, thymidine incorporation (an estimate of bacterial growth rate), and bacterial total direct counts. Results demonstrated that freeliving bacteria were responsible for most of the total bacterial production, which was correlated with temperature and particulate protein, but not with chlorophyll. In contrast, attached bacteria were much more active than free-living bacteria on a per-cell basis. Cell-specific thymidine incorporation by freeliving bacteria correlated with temperature and particulate protein quantity, whereas that by attached bacteria correlated with temperature, particulate protein quantity and particulate organic quality, as indicated by extent of phytoplankton degradation.