Termites are an important group of terrestrial insects that harbor an abundant gut microbiota, many of which contribute to digestion, termite nutrition and gas (CH 4 , CO 2 and H 2 ) emission. With 2200 described species, termites also provide a good model to study relationships between host diet and gut microbial community structure and function. We examined the relationship between diet and gut prokaryotic community profiles in 24 taxonomically and nutritionally diverse species of termites by using nucleic acid probes targeting 16Slike ribosomal RNAs. The relative abundance of domain-specific 16S-like rRNAs recovered from gut extracts varied considerably (ranges: Archaea (0^3%); Bacteria (15^118%)). Although Bacteria were always detectable and the most abundant, differences in domain-level profiles were correlated with termite diet, as evidenced by higher relative abundances of Archaea in guts of soil-feeding termites, compared to those of wood-feeding species in the same family. The oligonucleotide probes also readily distinguished gut communities of wood-feeding taxa in the family Termitidae (higher termites) from those of other wood-feeding termite families (lower termites). The relative abundances of 16S-like archaeal rRNA in guts were positively correlated with rates of methane emission by live termites, and were consistent with previous work linking high relative rates of methanogenesis with the soil (humus)-feeding habit. Probes for methanogenic Archaea detected members of only two families (Methanobacteriaceae and Methanosarcinaceae) in termite guts, and these typically accounted for 60% of the all archaeal probe signal. In four species of termites, Methanosarcinaceae were dominant, a novel observation for animal gut microbial communities, but no clear relationship was apparent between methanogen family profiles and termite diet or taxonomy. ß FEMS Microbiology Ecology 35 (2001) 27^36 www.fems-microbiology.org A. Brauman et al. / FEMS Microbiology Ecology 35 (2001) 27^36