~~Clostridium tetani and its related species C. tetanomorphum, C. cochlearium and C. lentoputrescens were examined for DNA-DNA homology and biochemical properties. Two distinctly different groups were included under the name of C. tetanomorphum: one was identical with C. cochlearium and the name C. tetanomorphum was applied to the other group with some amendment of biochemical properties. Comparison of the type strain of C. lentoputrescens with wild strains obtained from horse faeces indicated that the name C. lentoputrescens should be abolished as a later synonym of C. cochlearium. Liquefaction of gelatin and spore shape, which have been used as the important criteria for differentiation of C. tetani-related species, were genetically insignificant. I N T R O D U C T I O NThis study has investigated the relationships between Clostridium tetani-related species, i.e. C. tetani ( Bulloch et al. (1919) and the genus name Clostridium was applied to them in the first edition of Bergey's Manual of Determinative Bacteriology (Bergey et al., 1923). Since then, the name C. tetanomorphum has appeared in the Manual up to the seventh edition (Breed et al., 1957) but has disappeared from the eighth edition (Smith & Hobbs, 1974). Takahashi et al. (1967) have previously reported that two distinctly different groups were included under the name C. tetanomorphum. In the present study, tests for DNA-DNA homology, thermostability of DNA duplexes and biochemical properties were used to determine to which group the name of C. tetanomorphum should be applied. Furthermore, a possible identity of C. cochlearium and C. lentoputrescens has been re-investigated in the light of DNA-DNA homology determinations. METHODSStrains. These are listed, with the names as received, in Table 1. Strain K-130 was recovered from C. tetani HA-47 after heating at 100 "C for 30 min (Nishida et al., 1969). Non-toxigenic strains M-2, M-6 and M-14 have all the characteristics of C. tetani except toxicity.Preparation of DNA. Clostridium tetani HA-47, CN 655, 1110, 11 13, 1303, 61 14, 61 16, PO5 and 007, and C. tetani-like strain K-130 were grown at 37 "C in THPYG medium (pH 7.2) containing (%, w/v): trypticase (BBL), 0.5; heart infusion broth (Nissui, Tokyo, Japan), 0.5; polipeptone (Daigo, Osaka, Japan),