Members of candidate division OP3 were detected in 16S rRNA gene clone libraries from methanogenic enrichment cultures that utilized limonene as a carbon and energy source. We developed probes for the visualization of OP3 cells. In situ hybridization experiments with newly designed OP3-specific probes [OP3-565 and Eub-338(VI)] revealed abundant small OP3 cocci attached to larger cells. Syntrophic Deltaproteobacteria, OP3 cells, and methanogens affiliating with Methanoculleus and Methanosaeta formed the limonene-degrading community.