Mencl (1907) described cell nuclei in "symbiotic," not closely defined types of bacilli that he found in abundance in the digestive tract of the Küchenschabe, _Periplaneta_ (presumably _Blatta orientalis_). Unfortunately, he was more concerned about the morphology of the bacteria than the stated mutualistic relationship, so nothing is known of their physiology. The growth rates of _Periplaneta americana_ and _Blattella germanica_ were retarded when the insects were reared aseptically, which suggests that microorganisms normally found in the digestive tract supply certain necessary dietary constituents (Gier, 1947a; House, 1949). Noland et al. (1949) suggested that microorganisms in the digestive tract of _B. germanica_ synthesized riboflavin since the nymphs reared on a low riboflavin diet accumulated more of the vitamin than could have been ingested in the diet. However, Metcalf and Patton (1942) found little or no bacterial synthesis of riboflavin in _P. americana_. Noland and Baumann (1951) suggested that methionine, one of the amino acids essential for rapid growth of _B. germanica_, was synthesized by intestinal microorganisms in the insects. PROTOZOA It is probable that with few exceptions protozoa found in the digestive tract are not necessary for survival of the cockroach. However, very few experiments have been performed to determine the importance, if any, of these microorganisms to the host. Cleveland (1925) removed the protozoa from the cockroach (possibly _Periplaneta americana_) by oxygenation at 3.5 atmospheres. The ciliates _Nyctotherus_ and _Balantidium_, flagellates _Lophomonas_ and _Polymastix_, the amoeba _Endamoeba blattae_, and three unidentified protozoa were killed by this treatment, yet the insects lived normally after defaunation. Armer (1944) studied the effects of high-carbohydrate, high-fat, and high-protein diets, as well as starvation, on the intestinal protozoa (_Nyctotherus ovalis_, _Endamoeba blattae_, _Endolimax blattae_, _Lophomonas striata_, and _Lophomonas blattarum_) in _Periplaneta americana_. Starvation of the host lowered the incidence or eliminated most of the protozoa, but a high-carbohydrate diet maintained them at a relatively high level. _Lophomonas blattarum_ was eliminated by a high-protein diet, and practically eliminated by a high-fat diet. _Lophomonas striata_ was eliminated from some hosts that were kept on high-fat and high-protein diets. _Endamoeba blattae_ showed a decrease in infection rate when the cockroaches were maintained on high-fat and high-protein diets. The effects of diets on _Endolimax blattae_ were not uniform.