_Natural hosts._--_Blatta orientalis_, Europe (Hammerschmidt 1838, 1847; Bütschli, 1871); Egypt? (Galeb, 1878); England (Lankester, 1865; Lee, 1958); U.S.A. (Leidy, 1850a); U.S.S.R. (Yakimov and Miller, 1922; Sobolev, 1937; Sondak, 1935); Brazil (Travassos, 1929); China (Chitwood, 1932); Czechoslovakia (Groschaft, 1956). _Leucophaea maderae_, Brazil (Pessôa and Corrêa, 1926). _Periplaneta americana_, Brazil (Magalhães, 1900; Pessôa and Corrêa, 1926). U.S.A.: Texas (Todd, 1943); Kansas (Dobrovolny, 1933; Dobrovolny and Ackert, 1934); North Carolina (Hatcher, 1939); Iowa, North Dakota, Michigan (Hoffman, 1953). China (Chitwood, 1932). India (Basir, 1940). Czechoslovakia (Groschaft, 1956). England (Lee, 1958). _Periplaneta australasiae_, Brazil (Pessôa and Corrêa, 1926). _Polyphaga aegyptiaca_ (Linstow, 1878). Cockroaches (_Blatta orientalis_, _Blattella germanica_, and/or _Periplaneta americana_), U.S.A. (McAdow, 1931). Cockroach, Venezuela (Tejera, 1926). According to Hammerschmidt (1847) this worm may be found throughout the intestinal canal but especially in the small intestine. It is frequently found in adults and seldom in the nymphs. There were seldom more than 5 to 10 worms in one cockroach and female worms were found more frequently than males; the male worms were found only in winter and spring while the females were present at all times of the year. Bütschli (1871) stated that all stages from those just hatching to mature males and females are found. Yakimov and Miller (1922) found _H. diesingi_ in 50.8 percent of 124 _B. orientalis_ collected in Petrograd. Sobolev (1937) found 96 percent of _B. orientalis_ infected with _H. diesingi_ with a mean number of 5.6 and maximum number of 22 in one cockroach. Groschaft (1956) found 18 in one specimen of _B. orientalis_. Dobrovolny and Ackert (1934) found about 50 percent of 222 _P. americana_ infected with _H. diesingi_. Sondak (1935) found about 36 percent of 412 _B. orientalis_ infected with either or both _H. diesingi_ and _Leidynema appendiculata_. Two molts occur during development of the eggs; the first takes place outside the host resulting in a resting or infective stage. After the egg in the infective stage is eaten by the host, the second molt occurs before the egg hatches. Completion of the second molt and hatching perhaps are connected with ammonia present in the digestive tract; the ammonia seems to arise from the bacteria present in the gut. There appears to be a relationship between the intestinal bacteria of the cockroach and development and hatching of nematode eggs (Todd, 1944).