SUMMARY : Physiologically definable groups within the genus Lactobacillus are morphologically distinct. There are marked differences between homofermentative and heterofermentative strains and between physiological groups within the former. In recent years little use has been made of morphology in the classification of Lactobacillus. Much emphasis has been placed upon the pleomorphism of members of the genus, a state of affairs which is almost certainly due to inadequate control of growth conditions (cf. Smith, Gottschall & Wallgren, 1932). The existence of two main physiological types, homofermentative and heterofermentative is generally recognized, and species within these two types have from time to time been defined, sometimes upon adequate and sometimes upon less adequate criteria. A number of relatively simple tests has been adapted in this laboratory to the purpose of distinguishing certain specific groups, which correspond reasonably closely with those reported by Rogosa, Wiseman, Mitchell, Disraely & Beaman (1953), Sharpe (1955) and Wheater (1955a, b). These physiological tests will be the subject of a separate communication. METHODS The morphology and cytology of 221 strains of Lactobacillus were examined. Of these 23 were obtained by courtesy of the National Collection of Industrial Bacteria, Teddington, Middlesex, 3 were isolated from samples of yoghurt, and the remainder from human saliva and carious teeth. All were grown aerobically at 37" in tomato juice broth of the following formula: tinned tomato juice (filtered) 40 % (v/v), Oxoid peptone 1 yo (w/v), Difco yeast extract 0.5 yo (w/v), sodium acetate 1 Yo (w/v), glucose 0-5 yo (w/v), soluble starch 0-05 % (w/v), Tween 80 0.1 yo (v/v), salts A 0.5 yo (v/v), salts B 0-5 yo (v/v); pH value 6.8. The salt solutions A and B employed were those of Rogosa et al. (1953). The cell-form of the organisms was examined in cultures carried in & oz. screw-capped (bijou) bottles of this medium after 24 hr. incubation. Obser- vations were made upon unstained hanging-drop preparations, Gram-stained films, and preparations stained by Hale's (1953) method to demonstrate cell walls.