Microcosms consisting of estuarine sediment slurries were examined for their utility as models for assessing effects on microbial community structure and function. Data were obtained over a 2-week period to evaluate the reproducibility between individual microcosms and the variability between microcosm slurries and fresh sediment cores. Sulfate reduction rates in microcosm slurries did not differ significantly from rates for freshly collected sediment cores (p ี 0.05). However, the measured rates were more variable in microcosm slurries (SE ฯญ ฯฎ0.03-0.25 nM/ml/h) than in freshly collected sediments (SE ฯญ ฯฎ0.01-0.12 nM/ml/ h). Rates of dark CO 2 fixation in the microcosm slurries declined but were consistent with rates in freshly collected sediments (6.51 and 9.29 nM/ml/h on day 3, respectively). Relative abundances (RAs) of 16S rRNA determined for six specific phylogenetic assemblages of sulfate-reducing bacteria were reproducible among three microcosm replicates with Desulfovibrio spp. consistently in greatest abundance (RA ฯญ 8.61 ฯฎ 1.40, day 7). Total direct bacterial counts were not significantly different between freshly collected sediments and microcosm slurries (p ี 0.05). The results indicated that microcosms were both reproducible and representative of the field, and could thus provide a potentially useful tool for studies of microbial community response to perturbation.