It has proved difficult, in this laboratory, to identify the lactic acid bacteria isolated from silage according to the present system of classification. Keddie(1959) stressed this difficulty by his description of a number of lactobacilli isolated from silage, many of which were unidentifiable or apparently atypical of recognized species. Whittenbury(1960) found the same was true of streptococci and leuconostocs isolated from fresh grass and silage. This investigation, a series of studies on lactic acid bacteria isolated from fresh herbage and silage, aims at clarifying this situation. A number of named strains were also examined to assess the relationship of the silage organisms to the known species. This necessitated a general study of the various genera. This thesis is composed of six sections. The first concerns the reaction of various types of lactic acid bacteria to aerobic and anaerobic environments. The next four sections comprise studies on the classification of enterococci, pediococci, leuconostocs and heterofermentative lactobacilli. The final section is mainly of an applied nature and deals with aspects related to the production of silage.