The chironomid populations of a 6th order section (400 m) of the middle Garonne river were studied monthly from September 1996 to October 1997. For the first time the main channel of the river was investigated including two floodplain waters : a side arm and an oxbow. Two sampling techniques were used for drift and benthic collections. A total of 21 766 specimens (pupae and pupal exuviae) were sorted and 137 species were identified. Among these, 12 species are new for the fauna of France : their ecology and biogeography are given. The chironomid population of the backwaters is especially diverse and distinctive since the waterbody is disconnected from the main channel. The studied section appears to be transitional corresponding with the epipotamal-metapotamal zones. The embankment of the watercourse since 1958, which has isolated floodplain waters and increased the current velocity in the main channel, mainly explains the dual typologic status observed. The main channel is colonised by rheophilous Orthocladiinae whereas the backwaters shelter a mixed population of limnophilous Chironominae and potamobiont Orthocladiinae. This study demonstrates the urgency for preserving floodplain waters because of their species richness, their uniqueness and their faunal interaction with the main channel. The strong faunal contribution of floodplain waters highlights the topicality and the value in investigating the less disturbed potamal zones of large rivers to increase our knowledge on biodiversity, ecology and biogeography of chironomid populations.