The effects of porewater hydrogen sulfide concentrations on the feeding and tube-building activity of the subsurface, deposit-feeding polychaete, Clymenella torquata were experimentally determined in the laboratory. Porewater hydrogen sulfide concentrations were manipulated by injecting a buffered, isotonic sodium sulfide solution into the experimental chambers. Fecal material was collected and weighed and tube-building activity was monitored daily. Fecal production was negatively correlated with porewater hydrogen sulfide concentration during experiments conducted in June and July. Hydrogen sulfide concentrations greater than 1000 μM in June and 700 μM in July resulted in reduced fecal production. Fecal production was not correlated with porewater hydrogen sulfide concentration in experiments conducted in September and October, and some worms in the September experiment were surprisingly tolerant of porewater hydrogen sulfide concentrations as high as 3.8 mM. The lower tolerance of C. torquata to porewater hydrogen sulfide concentrations during the June and July experiments compared to the September and October experiments indicates that worms in the field became acclimated to increasing porewater hydrogen sulfide levels as temperatures increased. Tube-building frequencies were higher in worms in the June and July experiments than in the September and October experiments. In addition, experiments demonstrated that Clymenella is capable of modifying the porewater hydrogen sulfide concentration at depth in the sediment around its tube possibly by irrigation activities. These results suggest that porewater hydrogen sulfide concentrations may have significant non-lethal effects on the ecology of this species. Acquired tolerance to increased hydrogen sulfide concentrations may enable organisms to survive and feed under conditions in which other organisms cannot, making possible the exploitation of a niche used by relatively few benthic polychaetes. Porewater chemistry should be monitored and regulated during experiments on feeding rates and bioturbation in soft-bottom infaunal organisms.