The production of neuraminidase (EC 3.2.1.18) by a range of clostridial species was investigated with techniques previously developed to distinguish neuraminidase-negative and neuraminidase-positive strains of Clostridium perfringens (welchii). Large amounts of extracellular neuraminidase were produced by representative strains of C. perfringens and C. septicum in the test media. Under similar conditions, two strains each of C. chauvoei and C. tertium were found to produce small amounts of the enzyme. All of 12 strains of C. sordellii were clearly shown to produce neuraminidase, often in large amounts, but none of five strains of the closely related but non-pathogenic C. bifermentans had demonstrable neuraminidase activity. No neuraminidase was produced by C. novyi (oedematiens) types A-D (10 strains), C. tetani (6), C. botulinum types A, B, C or E (4), C. sporogenes (4), C. histolyticum (4) or by single strains of five other clostridial species.