A strain of anaerobic, syntrophic, propionate-oxidizing bacteria, strain LYPT (= OCM 661T; T = type strain), was isolated and proposed as representative of a new genus and new species, Smithella propionica gen. nov., sp. nov. The strain was enriched from an anaerobic digestor and isolated. Initial isolation was as a monoxenic propionate-degrading co-culture containing Methanospirillum hungateii JF-1T as an H2- and formate-using partner. Later, an axenic culture was obtained by using crotonate as the catabolic substrate. The previously described propionate-degrading syntrophs of the genus Syntrophobacter also grow in co-culture with methanogens such as Methanospirillum hungateii, forming acetate, CO2 and methane from propionate. However, Smithella propionica differs by producing less methane and more acetate; in addition, it forms small amounts of butyrate. Smithella propionica and Syntrophobacter wolinii grew within similar ranges of pH, temperature and salinity, but they differed significantly in substrate ranges and catabolic products. Unlike Syntrophobacter wolinii, Smithella propionica grew axenically on crotonate, although very slowly. Co-cultures of Smithella propionica grew on propionate, and grew slowly on crotonate or butyrate. Syntrophobacter wolinii and Syntrophobacter pfennigii grow on propionate plus sulfate, whereas Smithella propionica did not. Comparisons of 16S rDNA genes indicated that Smithella propionica is most closely related to Syntrophus, and is more distantly related to Syntrophobacter.