A component of Hamilton Harbour sediment prevented nitrous oxide (N 2 O) reduction in denitrification assays with a mixed population of endogenous bacteria and a pure culture (HH1) isolated from the sediment. A 5% (v/v) concentration of sediment in nutrient broth caused near maximum inhibition of N 2 O reduction. Sediment taken from a site closer to pollution sources (Site 906) was twice as inhibitory (as measured by N 2 O accumulation) as sediment from Site 910, further from pollution sources. N 2 O persistence was associated with the particulate sediment fraction only. Several heavy metals were tested at in situ concentrations, and ionic cadmium (Cd) and chromium (Cr) caused N 2 O accumulation. Ashed sediment did not cause N 2 O accumulation, but did decrease initial nitrate reduction rates with HH1.