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The effect of feeding to cockroaches the antimicrobial agent metronidazole, which acts specifically against anaerobes, was assessed by light and scanning electron microscopy, bacteriological examination, and methane formation. Types of organisms and total numbers were greatly reduced from controls. The health of mature adults was unaffected, but stunting occurred in subadult animals maintained on the antibiotic from hatching. The intracellular bacteria of the fat body were not affected by the drug. The results are discussed with respect to a proposed microbe-host extracellular symbiosis.Studies of the alimentary canal of the cockroach indicate the presence of an elaborate hindgut microflora (7). Many of the attached microbes and unusual morphological forms have been refractory to isolation. As an alternative to the isolation and characterization of all the anaerobes possible, we have approached the problem of interactions of the hindgut flora with the animal via the possibilities of transport of microbial products across the hindgut wall (J. W. Bracke and A.