Intestinal nematodes of the family Thelastomatidae have no apparent pathological effect on their cockroach hosts. Galeb (1878) has shown experimentally that oxyurids eat the same food as the host insect and that if one starves them, by withholding food from the host, the oxyurids die and disappear. In other words, these worms are not parasites, in the sense that we use the term in this paper, but commensals. Dobrovolny and Ackert (1934) stated that "all observations seemed to indicate that the health, fertility and activity of the heavily infested cockroaches were comparable with those of the non-parasitised specimens." Very few papers have dealt with the ecology of the oxyurid parasites of cockroaches. According to Galeb (1878), usually one species of nematode is found in a single cockroach, but sometimes two species live together in the same host (e.g., in _Blatta orientalis_ and _Polyphaga aegyptiaca_) where they compete for food. Galeb claimed that _Hammerschmidtiella diesingi_ would replace _Leidynema appendiculata_; he observed that _H. diesingi_ surpassed _L. appendiculata_ in numbers and the latter became uncommon in the intestines of the cockroaches. On the other hand, Sobolev (1937) found that 48 percent of his oriental cockroaches were infected with both of the above species of nematodes. The average number of both species was 7.5, and the maximum number was 97; the mean number of _H. diesingi_ was 5.1 and the maximum 64; the mean number of _L. appendiculata_ was 2.4 and the maximum 33. More than 40 nematodes were found in each cockroach of 1.3 percent of those examined. These results apparently contradict Galeb's conclusions inasmuch as the number of each species in mixed infections was essentially the same as the number found in cockroaches infected by only one species (see pp. 195 and 197). Dobrovolny and Ackert (1934) found that 29 percent of 222 _Periplaneta americana_ contained both of the above species of nematodes; whereas 40 percent contained _L. appendiculata_ only, and 21 percent contained _H. diesingi_ only. The infestation ranged from 1 to 36 worms per cockroach with averages of 3.8 per male, 5.1 per female, and 2.7 per nymph.