1. The importance of microhabitat characteristics for macroinvertebrate microdistribution was studied within a reach of themain channel of the Upper Rhône River (France). 9 locations (3 on the left hand side, 3 across the middle, and 3 on the righthand side) were sampled twice on two dates (as duplicates in June and July 1982). From 19 variables, including substratum andhydraulic parameters, 14 were selected to describe the physical habitat of each location. 47 macroinvertebrate species were collectedby means of dredging with a modified Rallier du Baty dredge. The habitat-fauna co-structure was studied using co-inertiaanalysis.2. The river reach was transversally asymmetrical : shallow on one side (up to 2.80 m) with coarse sediments rich in periphyton,and much deeper on the other side (up to 4.80 m) with a gravel bottom rich in organic particles. In the middle of thechannel these conditions were intermediate, but the current was stronger. Velocity and the other hydraulic parameters variedindependently of the substratum characteristics. This independence is a consequence of old man-made impoundments (inheritedstructures) and perhaps of present paving of the substratum also.3. The macroinvertebrate fauna was closely linked to this transverse asymmetry of the reach, with the highest richness andabundance to the shallow side of the channel and the lowest species richness in the rapid central part. The most significant faunisticdifferences were linked i) to water depth, acting on both hydraulic parameters and periphyton (by light transmission), ii)to the inherited old bottom substratum, and iii) to a lesser extent, to the present current velocity.