Brittle failure has been observed in polyoxymethylene during long-term low-level tensile loading at elevated temperatures. It is argued to be associated with slow crack growth via the breakdown of the localized planar fibrillar damage zones that form under these conditions. This phenomenon has been characterized using notched compact tension specimens tested under various static loads and at different temperatures. The specimen lifetime at a given load is found to decrease strongly with increasing temperature and to increase with molar mass at a given load and temperature. The associated crack-tip fibrillar damage zones are shown to arise from the breakdown of more localized microfibrillar deformation zones, which in turn result from interlamellar cavitation in the early stages of tensile deformation.