Aim To assess temporal changes in gammarid distribution in Brittany and microhabitat-use overlap between the endangered endemic Gammarus duebeni celticus Stock & Pinkster, 1970, the expanding natives G. pulex (Linnaeus, 1758) and Echinogammarus berilloni (Catta, 1878), and the introduced G. tigrinus Sexton, 1939. Location Brittany and adjacent regions in western France. Methods The spatial and temporal patterns in distribution of gammarids at the scale of Brittany were studied using 351 sites. Longitudinal distributions (from the source to the estuary of the river) and microhabitat-use (substratum type and water velocity) were also considered in selected rivers. Results At the regional scale, all species occurred together less often than expected statistically, with significant deviations from expected for G. pulex vs. both G. duebeni celticus and G. tigrinus, and for E. berilloni vs. both G. duebeni celticus and G. tigrinus. However, at the microhabitat scale, E. berilloni occurred significantly more often than expected with the endemic G. duebeni celticus, and this appears to be due to similar substratum and water velocity preferences, although at both the regional and microhabitat scales E. berilloni prefers wider streams than G. duebeni celticus. This study reveals a decline in the endangered G. duebeni celticus since 1970. Main conclusions The longitudinal and local distributions of G. duebeni celticus, and the higher-than-expected co-occurrence of the species with G. pulex, suggest that the decline of the endemic species may be due to changes in the environment and/or interference from native G. pulex, which is expanding its range in Brittany. The results are discussed as regards to the consequences for regional biodiversity.