In previous paper, the authors have reported that the bacterial decomposition or synthesis scemed to be the main cause of the marked variations observed on the vitamin content in spoilage of marme aniimals. To ascertain this respect, aerobic and, anaerobic bacteria were isolated from jack mackerel, baby clam, sardine, mackerel-pike and goby meats and their abilities of decomposition or production were tested (from Table 2 to 4). These bacteria were isolated in an usual manner and some of them were classified after the Bergey's Manual of Determinative Bacteriology. The isolated bacteria were incubated with vitamin B12 in the tests for decomposing activity and without it for producing power. After incubation, culture media and bacterial cells were treated and extracted at 90°C for 10 mins. and analysed for the survival vitamin by the Euglena method. On the other hand, fifty-six species of the authentic various bacteria were examined on their activity on the vitamin in the same manner above mentioned and the results are summarized in Table 5. Some of these bacteria decomposed the vitamin remarkably, wherear othes synthesized it or remained unconcerned, and the behavior of bacteria for vitamin B12 seemed to be species-specific. However, it is noticeable that Achromobacter and Pseudomonas, common fish-spoiling bacteria, Escherichia coli and Bacillus subtilis contain more decomposing species and producing bacteria less.