Bacteria bound to particles can be ingested by marine calanoid copepods, thus potentially providing a direct trophic link between bacteria and metazoa provided the microbes are digested by the animals. If some bacteria survive gut passage, they might play a role in fecal pellet solubilization and fragmentation (via ectoenzymes) and decomposition. We examined whether Calanus pacificus could assimilate bacterial biomass and whether a significant fraction of the ingested bacteria passed into the fecal pellets alive. We fed adult female copepods on freeze-thawed diatoms (Cylindrotheca fusiformis and Chaetoceros sp.) which had been colonized by natural populations of seawater bacteria labeled with 3H-leucine. The copepods retained 26 to 31 % of the label 43 h after ingestion and subsequent gut clearance, indicating that a substantial fraction of bacterial biomass bound to particulates of appropriate size can be directly ava~lable to this copepod as a source of material and energy. Bacteria were present at very high concentrations (10' to 101° ml-l) in the fecal pellets of copepod fed on bacterized food, but were absent in the fecal pellets of copepods fed axenic food. When fecal pellets freshly ejected by copepods fed bacterized food were incubated in seawater, bacteria abundances in them increased, yielding minimum specific growth rates of 0.08 h-' (generation time g = 8.7 h) at 16 "C and 0.19 and 0.20 h-' ( g = 3.6 and 3.5 h) at 22 "C. Since bacteria grew within the fecal pellets, at least some of the bacteria we found in the freshly egested fecal pellets must have survived gut passage In order to determine whether bacteria imparted aminopeptidase activity to copepod fecal pellets, the enzyme activity of fecal pellets produced on uncolonized and colonized freeze-thawed diatoms was compared. This comparison showed that bacteria in the fecal pellets account for 68 to 84 %of the aminopeptidase activity of fecal pellets, indicating a potential role for the interior bacteria in fecal pellet solubilization and fragmentation. We conclude that feeding of these copepods on attached bacteria not only serves to link the microbial loop with the grazing food chain but, because of the passage of viable bacteria into the fecal pellet, may also influence fecal pellet degradation.