Previous studies of the endogenous microbiota in patients with ulcerative colitis (UC) have not taken bacterial activity into account, yet bacteria with high transcriptional activity might have a more important pathophysiological role than inactive bacteria. We therefore analyzed the biodiversity of active bacteria in the fecal microbiota of UC patients, in comparison with that of healthy subjects. Feces were collected from nine patients with active UC and from nine healthy controls. Total DNA and RNA were extracted, and 16S ribosomal DNA and RNA were amplified by PCR and reverse transcription-PCR, respectively. Amplification products were compared by means of temporal temperature gradient gel electrophoresis (TTGE). Bands of interest were excised, sequenced, and identified by comparison with the GenBank database (NCBI). The dominant-species diversity based on RNA-derived TTGE profiles was significantly lower for UC patients than for healthy controls (P = 0.01). The mean similarity index between the "present" and "active" microbiota was 74% ± 18% for UC patients. Comparison of the individual "active" microbiota identified a band that was present for eight UC patients and only two controls (89% versus 22%; P = 0.008). The band was sequenced for 6 patients and always corresponded to Escherichia coli. The biodiversity of active bacteria in the dominant fecal microbiota of patients with UC is lower than that of healthy subjects. E. coli is more represented in the active microbiota of UC patients. The possible pathophysiological role of this difference remains to be determined.