Thermophilic (strain GOMI-1(T)) and mesophilic (strain KOME-1(T)) strains were isolated from two different cultures of propionate-degrading consortia obtained from thermophilic digester sludge and rice paddy soil, respectively. The two strains were non-spore-forming, non-motile and Gram-negative. Both strains were obligately anaerobic micro-organisms, showing multicellular filamentous morphotypes more than 100 mum in length. The cell width for strain GOMI-1(T) was 0.2-0.4 mum and that of strain KOME-1(T) was 0.4-0.6 mum. Strain GOMI-1(T) could grow at 45-65 degrees C with a pH range of 6.0-7.5 (optimum growth at 55 degrees C, pH 7.0). The temperature range for growth of strain KOME-1(T) was 30-40 degrees C and the pH range was pH 5.0-8.5 (optimum growth around 37 degrees C, pH 7.0). Yeast extract was required for growth of both strains. Strain GOMI-1(T) was able to grow with a number of carbohydrates in the presence of yeast extract. In yeast extract-containing medium, strain KOME-1(T) could utilize proteins and a limited range of sugars for growth. The G+C contents of the DNA of strains GOMI-1(T) and KOME-1(T) were respectively 54.7 and 57.6 mol%. Major fatty acids of strain GOMI-1(T) were C(16 : 0), C(14 : 0) and iso-C(15 : 0), whereas those of strain KOME-1(T) were iso-C(15 : 0), anteiso-C(15 : 0) and C(14 : 0). Based on comparative analysis of 16S rRNA gene sequences of strains GOMI-1(T) and KOME-1(T), the strains were placed in different phylogenetic positions in the class Anaerolineae of the bacterial phylum Chloroflexi. Their phenotypic and genetic traits strongly supported the conclusion that the strains should be described as two independent taxa in the class Anaerolineae. Hence, we propose the names Bellilinea caldifistulae gen. nov., sp. nov., and Longilinea arvoryzae gen. nov., sp. nov., for strains GOMI-1(T) and KOME-1(T). The type strains of Bellilinea caldifistulae and Longilinea arvoryzae are respectively GOMI-1(T) (=JCM 13669(T) =DSM 17877(T)) and KOME-1(T) (=JCM 13670(T) =KTCC 5380(T)).