Paraformaldehyde-resistant starchfermenting bacteria in "starch-base" drilling mud. Appl. Microbiol. 10:418-421. 1962-Starch-fermenting bacteria were found in each of 12 samples of nonfermenting starchbase drilling mud examined. Of the 12 samples, 3 contained very active starch-fermenting gram-positive spore-bearing bacilli closely resembling Bacillus subtilis. Similar active starch-fermenting bacteria were found in fermenting starch-base drilling mud and in corn starch and slough water used to prepare such mud. The active starch-fermenting microorganisms completely hydrolyzed 1 % (w/v) corn starch within 24 hr at 37.5 C. The active starch-fermenting bacteria isolated from fermenting drilling mud were capable of surviving 12 hr of continuous exposure to 0.1 % (w/w) paraformaldehyde or 1 hr of continuous exposure to 0.5 % (w/w) paraformaldehyde, with no diminution in starch-fermenting ability. The same organisms fermented starch after 3 hr of continuous