Abstract Under anaerobic conditions, the tapeworms, Hymenolepis diminuta and Oochoristica symmetrica, ferment glucose and galactose, at least partially, to acidic end-products. The fermentation of mannose and xylose by the former and of mannose by the latter is somewhat equivocal. The acanthocephalan, Moniliformis dubius, ferments glucose, galactose, mannose, fructose and maltose to acidic catabolites. No evidence of the acidic fermentation of a variety of other carbohydrate substrates by these helminths has been found. Phlorizin, non-competitively, inhibits the fermentation of glucose by H. diminuta but has no effect on the fermentation of glucose, mannose, fructose, and maltose by M. dubius. Dibromophlorizin inhibits glucose and galactose fermentation by H. diminuta but has no influence on sugar fermentation by M. dubius. These glucosides inhibit galactose fermentation but not glucose fermentation by O. symmetrica. The fermentation of glucose and fructose by M. dubius is inhibited by p-chloromecuribenzoic acid but not by iodoacetic acid. Quinacrine inhibits the fermentation of glucose and galactose by O. symmetrica and glucose fermentation by H. diminuta; whereas, acriflavine inhibits glucose fermentation but not galactose fermentation by these cestodes.