In fish muscle press juice simulating the surface and the interior of muscle, there is an aerobic environment in the surface layer and an anaerobic environment in the body of the liquid. The Eh potential of the former is about 0.3 volts and of the latter from −0.5 to −0.10 volt.It is found that the bacterial population proliferating at 2 °C. is chieflyAchromobacter, which can be divided into two groups, obligate aerobes and facultative anaerobes. Only the latter group, which is capable of growth in the interior or surface, is responsible for the reduction of trimethylamine oxide with the evolution of trimethylamine. Since the initial total count is made up of a large number of obligate aerobes or non-oxide reducers it is obvious that the total bacterial population cannot be related to trimethylamine production. The appearance of this base therefore may be taken to indicate a bacterial population which is in excess of that responsible for its production.Molecular oxygen at surface exercises a trimethylamine oxide sparing effect. In practice, however, this effect is not significant from the point of view of the freshness test in the sense of Beatty and Gibbons.