The boundary between freshwater and estuarine ecosystems at the extreme head of estuaries has received little attention from benthic ecologists. This short communication addresses this research caveat, presenting preliminary data on the spatial and temporal (seasonal) variation in the composition of benthic macroinvertebrate communities across the transition zone at the head of a small British estuary. Cluster analysis and multidimensional scaling ordination demonstrated that a distinct community occurred at a point just preceding the recognised transition between fresh and brackish waters (salinity <0.2PSU). This transition community contained a mixture of freshwater and estuarine taxa and, despite seasonal migrations at the head of the estuary, community structure remained significantly distinct from that at adjacent sites throughout the year. These preliminary data suggest that the community dynamics across the transition zone show complex patterns and may offer an important research opportunity in aquatic ecology.