Content: The early response to environmental disturbance and antropogenic pressure by estuarine fishes make them good indicators of changes, especially in regard to water quality. From 2008 to 2010 sampling surveys of demersal assemblages in the Basque estuarieswere carried out three times per year (spring, summer and autumn). The results obtained have allowed to describe the demersal communities (fish and crustaceans) established in these estuaries. In addition, the ecological status of the demersal community was assessed in each estuary, by the AFI index (AZTI’s Fish Index) and the response of the demersal community to human pressures was analyzed. Moreover, a crustacean alien species has been detected and identified.