Members of the bacterial genus Fibrobacter have long been considered important components of the anaerobic cellulolytic community in the herbivore gut, but their presence and activity in other environments is largely unknown. In this study, a specific polymerase chain reaction (PCR) primer set, targeting the 16S rRNA gene of Fibrobacter spp., was applied to community DNA from five landfill sites followed by temporal thermal gel electrophoresis (TTGE) analysis of cloned amplification products. Phylogenetic analysis of clone sequences indicated the presence of novel clusters closely related to the genus Fibrobacter. There are two named species, Fibrobacter succinogenes and F. intestinalis, and only two of the 58 sequenced clones were identified with them, and both were F. succinogenes. The clone sequences from landfill were recovered in five distinct clusters within the Fibrobacter lineage, and four of these were novel. Quantitative PCR (qPCR) assays of reverse-transcribed community RNA from landfill leachates and rumen fluid samples indicated that the abundance of Fibrobacter spp. relative to total bacteria varied from 0.2% to 40% in landfill, and 21% to 32% in the rumen, and these data demonstrate that fibrobacters can be a significant component of the microbial community in landfill ecosystems. This is the first evidence for Fibrobacter spp. outside the gut ecosystem, and as the only cultivated representatives of this group are actively cellulolytic, their diversity and abundance points to a possible role in cellulose hydrolysis in landfill, and perhaps other anaerobic environments also.