We compared freshwater mussel assemblages (Bivalvia: Unionidae) in a developing oxbow (old channel) and a newly-cut (new) channel of the Neosho River, Kansas, during 2001. We sampled mussels at approximately 1.5 km intervals, totaling 19 sites in the old channel and seven in the new, by groping substrate from bank to bank in a zigzag fashion along a 100 m reach. We found 1239 live individuals of 20 mussel species in the old channel and 20 live individuals of six species in the new channel. Mussel abundance, diversity and current species richness were greater in the old channel than in the new. Abundance, diversity and historic and current species richness all declined significantly from upstream to downstream in the old channel. Historic richness was greater than current richness in the old channel, but both decreased from upstream to downstream at the same rate. The loss of historic species from the old channel was no different from that in the rest of the Neosho River in Kansas; however, a further decrease in lotic species will likely occur as the channel continues to evolve into an oxbow lake. Canonical correspondence analysis illustrated a substrate compaction, substrate composition and depth gradient in the old channel, with three species in flowing waters with sand and gravel substrate and all other species in still waters with silt substrate. Information regarding habitat use by freshwater mussels is important in designing surveys and recovery plans for rare species (Strayer, 1993).