When sterile cod muscle was inoculated with pure cultures of bacteria and held at 3 °C., it was found that the common spoilage odours were rapidly produced by Pseudomonas, Proteus, Achromobacter and Serratia species. Micrococcus and Flavobacterium species produced milder forms of spoilage and much more slowly. Mesophilic species which rapidly decompose fish at 25 °C. produce no apparent spoilage at 3 °C. even when the initial inoculum contains a very large number of cells.