The eggs of some helminths pass unharmed through the guts of cockroaches that serve as vectors of these ova and have no effect on the insect. However, helminths that are secondary parasites in cockroaches damage the insect to varying degrees depending upon the extent of the infection. Thus the larvae of _Moniliformis moniliformis_ pass through the gut wall and some may become embedded in the fat tissue (Moore, 1946). First stage larvae of _Oxyspirura mansoni_ also burrow through the midgut wall into the fat body; Sanders (1929) believed that _Pycnoscelus surinamensis_ could be killed if at one time a sufficient number of migrating larvae of _O. mansoni_ penetrated the cockroach's intestinal wall. _Gongylonema neoplasticum_ migrates through the digestive tract and encysts in the muscles of the thorax and legs of the host (Fibiger and Ditlevsen, 1914). Infective larvae of _Protospirura muricola_, after hatching from ingested eggs, pass through the cockroach's gut wall and encyst mainly in the thorax, around the crop, and at the bases of the large muscles of the prothoracic legs (Foster and Johnson, 1939). It is probably generally true that nematodes which are secondary parasites in cockroaches do some damage to the host's intestinal tract at least, and they probably also damage other organs in which they may encyst. Cockroach tissues may react defensively to infections by parasitic nematodes. For example, encysted third-stage larvae of _Physaloptera turgida_ have been found enclosed in a thin, brown, chitinous substance that was undoubtedly deposited by the tissue of the cockroach (Alicata, 1937). Cysts of similar appearance have been found in cockroaches infected with _Physaloptera rara_, _P. maxillaris_, _P. hispida_ (Petri, 1950; Hobmaier, 1941; Schell, 1952), and _Gongylonema pulchrum_ (Schell, 1952a); in the latter species the deposit eventually completely surrounded the nematode larva which was killed and "chitinized." Apparently these pigmented cysts surround unhealthy or dead larvae and are secreted as a defensive mechanism by the host (Schell, 1952a). Oswald (1958) has reported finding similar pigmented cysts in _Blatta orientalis_ and _Periplaneta americana_ that were experimentally infected with _Rictularia coloradensis_. Our classification of the helminths follows Hyman (1951, 1951a). HELMINTHS FOR WHICH COCKROACHES SERVE AS PRIMARY HOSTS Phylum ASCHELMINTHES Class NEMATODA Order MERMITHOIDEA Family MERMITHIDAE =Undetermined mermithids= _Natural Hosts._--_Ectobius pallidus_, U.S.A., Plymouth, Massachusetts (Roth and Willis, 1957): This mermithid lies coiled in the body cavity of the host and one end may extend into the thorax. Apparently, the host is eventually killed and the worms may leave the cockroach ventrally between the thorax and abdomen (pl. 29, A) or thorax and head.