Microbial and meiofaunal abundances and bacterial production on sediment tubes produced by the polychaete Capitella cap~tata (Type I ) were examined in laboratory microcosms. Protozoa densities and bacterial production were significantly higher on tubes than in equivalent volumes of sediment taken adjacent to the tubes. Bacterial densities on tubes and in control sediments were not significantly different. Total meiofauna densities were significantly higher in controls due to the absence of nematodes on the tubes. The epibenthic copepod Tisbe holothuriae was the only meiofauna species found on the tubes, but their densities were not significantly greater than control populations. This work suggests that sediment tubes constructed by C. capitata are sites of enhanced bacterial activity caused, in part, by protozoans grazing on bacteria. Most research on tube-dwellings of macrobenthos has focused on structures constructed by deep-dwelling equilibrium species, rather than on tubes produced by pioneer, surface deposit-feeders (Aller & Yingst 1978, Reise 1981). Biogenic structures of equilibrium species have received much more attention than structures produced by pioneering species due, in part, to the greater ability of bioturbating organisms to influence sediment biogeochemistry, and to alter the distribution and abundances of microbes and meiofauna. However, few investigators have considered the effects of pioneering benthos and their tubes on the dynamics of microbial and meiofaunal populations (Rhoads et al. 1978, Reise 1981). In flume studies (Rhoads et al. 1978), dense tube aggregations of the capitellid polychaete Heteromastus filiformis led to increased stability of sediments. Stimulation of bacterial growth, Rhoads et al. (1978) proposed, may have been one explanation for this result. This report describes microbial and meiofaunal abundances and bacterial production, on tubes constructed by the 'Present address: Australian Institute of Marine Science, D. M. B. No. 3, Townsville M. C., Queensland 4810, Australia O Inter-Research/Printed in F. R. Germany capitellid polychaete Capitella capitata (Type I ) in laboratory microcosms. Sediment tubes of Capitella capitata were examined in 0.25 m2, 14 cm deep, plastic tanks that received 200 m1 min-l of flow-through, 1 pm filtered, temperature-regulated (20 C ? 1 C) seawater (24 + 2 %o). Mixed cereal was used as detritus source and was added at a rate of 100 mgN m-2 d' ( = 4 g dry weight m-2 d-l). The seawater was continually aerated. Each tank was layered to a depth of 5 cm with autoclaved, fine-grain ( 0.05) different. Total meiofauna densities were significantly higher in control samples due to the absence of nematodes on tubes. Table 1. Microbial and meiofaunal abundances, and bacterial activity, on and adjacent to tubes of Capitella capitata. Values are given as mean f l SE of 5 replicates. Protozoa and meiofauna densities are expressed as number of individuals 18 mm-3 sediment