O. TRESSE, F. MOUNIEN, M.-J. LE ́VESQUE AND S. GUIOT. 2004.Aims:To understand the microbial ecology underlying trichloethene (TCE) degradation in a coupled anaerobic/aerobic single stage (CANOXIS) reactor oxygenated with hydrogen peroxide (H2O2) and in an upflow anaerobicsludge bed (UASB) reactor.Methods and Results:The molecular study of the microbial population dynamics and a phylogeneticcharacterization were conducted using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). In both reactors, TCE had a toxic effect on two uncultured bacterial populations whereas oxygen favouredthe growth of aerobic species belonging to Rhizobiaceae andDechloromonas. No methanotrophic bacteria weredetected when targeting 16S rRNA gene with universal primers. Alternatively,pmogene encoding the particulatemethane monooxygenase ofMethylomonassp. LW21 could be detected in the coupled reactor when H2O2wassupplied at 0Æ7gO2l 1Rday)1.Conclusions:Methylomonassp. LW21 that could be responsible for the aerobic degradation of the TCE by-products is not among the predominant bacterial populations in the coupled reactor. It seems to have beenoutcompeted by heterotrophic bacteria (Rhizobiaceae andDechloromonassp.) for oxygen.Significance and Impact of the Study:The results obtained show the limitations of the coupled reactorexamined in this study. Further investigations should focus on the operating conditions of this reactor in order tofavour the growth of the methanotrophs.