Polybrominated diphenylethers (PBDEs) are a class of commonly used flame retardants. Through product breakdown, these compounds may become concentrated in the environment. This study investigated the presence and anaerobic breakdown of decaBDE (a fully brominated PBDE congener) in Hamilton Harbour sediment over a laboratory incubation period of 189 days. The detected background concentration of this compound in the harbour sediment is 15.4 ng/g sediment (dw). Over the entire incubation period, there was no significant degradation of decaBDE; yet, fluctuations in decaBDE concentration throughout the incubation time period may be attributable to the presence and initial consumption of organic carbon by microbial communities in the sediments. Microbial identification did not show the presence of known PCB or PBDE degraders. Due to the heterogeneity of Hamilton Harbour sediments, however, it is highly possible that these communities thrive in other parts of the harbour sediment not sampled for this particular study.