The amount of NaNO2 formed by reduction of NaNO3 by growing heterotrophic fish spoilage bacteria depended largely on the pH of the culture medium used. Thus NaNO2 formation was strongly inhibited in media controlled at acid pH, but amounts as high as 6,950 p.p.m. were formed when the pH was permitted to rise unchecked. NaNO3, in presence or absence of 200 p.p.m. of NaNO2, had either a very trivial effect, or no effect, on the growth of fish spoilage bacteria in broth medium or fish muscle, and it was either not, or but feebly, reduced in fish muscle under the experimental conditions.