The upstream sector of the "Plaine de la Bassée", a middle reach of the Seine River, exhibits a typical sub-natural character with many meanders and backwaters. Most of these backwaters are directly connected to the main channel by just one connection and have a lentic character.To examine the macrohabitat influencing the young-of-the-year (YOY) fish assemblages, we studied 20 of these backwaters. At each site, we sampled the YOY fish assemblages and collected 12 macrohabitat variables. Fish were collected by electrofishing using Point Abundance sampling (PAS) in August and September 1995. The macrohabitat variables included morphology (surface, average depth and degree of connectivity), location in the upstream-downstream gradient, biological activity (total organic carbon and riparian cover) and bank habitat (% of shelter, % of vegetation, % of gravel steep bank, % of silt beach, and % of silt steep bank). We captured 9061 YOY fishes representing 21 species, which were divided into reproduction guilds according to BALON's classification. Step-by-step regressions were used to develop linear models between the density of each reproduction guild and the macrohabitat variables. Results differ between the reproduction guilds. Phytophilics have the strongest relationship with the environmental variables (r(2)=0.8), whereas ostraphilics show no relationship. Two macrohabitat variables, the backwater location in the upstream-downstream gradient and the degree of isolation, show a great effect on the YOY fish assemblages. These variables are included in three models.