We applied the seawater soft-agar microcosm approach in order to reveal the effects of sodium sulfide on burrowing activity of Capitella sp. I and on bacterial respiratory activity on mucus-lined burrow. The presence of sodium sulfide up to 7 mM in the soft-agar was not detrimental to the burrowing activity and the survival of the worms, but seemed to enhance the secretion of mucus from the worms. Addition of INT into overlying water (0.01% v/v) of the soft-agar microcosms containing the same concentrations of sodium sulfide caused intensive deposition of INT-formazan in the water, on the mucus over soft-agar surface and on the burrow lining. Both image analyses and spectrophotometric determination of the red-color INT-formazan showed a marked increase in the presence of 3.5 mM sulfide. From these results, we concluded that sulfide can promote burrowing activity of Capitella sp. I and thus enhance the bacterial respiratory activity on the burrow lining. These findings will help clarify the possible interaction between Capitella worms and sulfur-oxidizing bacteria in organically-enriched, sulfide rich sediment in natural environments.