Leuconostocs from herbage and grass silage were compared with strains from culture collections. The organisms were divided into sections, on ability to form detectable amounts of H2O2 from glucose and dextran from sucrose, and into groups by various tests of which the most useful were catalase activity, ability to grow at various temperatures, manner of growth in glucose soft agar and action on pentoses. The satisfactory division of all these organisms into species on ability to ferment pentoses and sucrose and to form dextran from sucrose, the basis of Hucker and Pederson's (1931) classification, was not possible. No useful alternative scheme was apparent. The preference of one group of organisms for aerobic conditions coupled with their ability to form catalase when provided with haem compounds, suggests that leuconostocs may have evolved from organisms possessing a respiratory pathway.