1. In a study of 129 cases of appendicitis in children, we have found that in nineteen of them (15 per cent.) the appendix contained Oxyuris vermicularis or Trichocephalus trichiura. 2. The oxyuris is much the commoner of the two parasites, occurring seventeen times, while the trichocephalus was found only twice. 3. In fifteen out of the nineteen cases, the parasites were associated with a non-suppurative catarrhal type of appendicitis. If the four cases of catarrhal oxyuris appendicitis of adults be also included, it may be said that 83 per cent. of the cases of appendicitis associated with parasites have been of the catarrhal type. In the four remaining cases, the appendix was gangrenous. The cellular changes noted in the catarrhal cases have been distension of the crypts with mucus, hyperplasia of the lymph follicles, and in some cases, the presence of a few leukocytes in the muscularis and serosa. 4. In thirteen of the nineteen cases (70 per cent.), there have been noted, in addition to the cellular changes, certain lesions in the mucosa, unquestionably produced by the parasite. Of the six cases in which no specific lesions were demonstrated, the extensive gangrene in three of them might have obliterated specific lesions had they been present. Specific lesions were not demonstrated in any one of the four adult cases. 5. The oxyuris injures the mucosa by breaking through the lining epithelium and burrowing into the lymph follicles for a greater or less distance. Its invasion is usually accompanied by extravasation of blood into the surrounding lymphoid tissue, and its escape through the ruptured mucosa. In some cases, the oxyuris produces hemorrhagic ulcers in the mucous membrane, the edges of which are undermined and the bases formed by the submucosa. A characteristic feature of the oxyuris lesions is the absence of inflammatory reaction about them, except in those cases where there is secondary bacterial infection. 6. In some instances, the oxyuris may become encapsulated in the mucosa with fibrous tissue, and itself undergo hyaline degeneration. Such a process constitutes a healed oxyuris lesion. 7. The trichocephalus attacks the mucous membrane of the appendix in a manner similar to that described for the oxyuris, and there is the same absence of inflammation about the invading parasite. The trichocephalus, however, instead of invading a lymph follicle, shows a tendency to burrow along just beneath the columnar epithelium. 8. The conclusion to be drawn from these facts is that oxyuris and trichocephalus, when they occur in a diseased appendix, are, in most cases, the existing cause of the pathological changes found.