In-situ measurement of chemolithotrophic and some heterotrophic microbial activities were made in the immediate vicinity of actively discharging hydrothermal vents of the Galfipagos Rift region at depths of 2 500 to 2 600 m. The CQ-assimilation or chemosynthesis productivity in the emitted vent waters, freshly mixed with oxygenated ambient seawater of 2 ~ was minor compared to the bacterial biomass produced within the subsurface vent system prior to emission. Uptake of acetate and glucose indicated the presence of mixotrophic or facultatively chemolithotrophic bacteria in the emitted vent waters in agreement with isolations. Demonstration of ribulose bisphosphate carboxylase and phosphoenol pyruvate carboxylase in cultures of thiobacilli isolated from these vent water supports the notion that chemoautotrophic sulfur-oxidizing bacteria are one of the sources of primary production in the form of particulate organic carbon for filtering organisms in the deep sea hydrothermal environment. The rates of bacterial metabolic activities in emitted vent water are too low for the amount of invertebrate biomass and the rate of its growth and maintenance. Therefore, the larger portion of chemosynthetic sustenance of deep sea vent ecosystems appears to be based on symbiotic associations between bacteria and invertebrates and on surface attached bacteria.