Bacterial counts, trimethylamine and trimethylamine oxide concentrations, and pH in gutted cod stored in melting ice show that spoilage originates in the belly cavity. Bacteria do not penetrate the muscle until spoilage is well advanced, but the substrates, trimethylamine oxide, etc., diffuse outward to the surface and the bacterial products diffuse rapidly into the muscle. Leaching by the ice water lowers the concentration of trimethylamine oxide at the skin. A gradient of high trimethylamine concentration in the belly cavity, decreasing through the muscle to low values at the skin, rapidly develops.