In a prospective study of 132 cases of gastroduodenal ulcer, proven by endoscopic biopsy, there were 59 in which fungi were cultered (44.7%). There was no significant difference in incidence between gastric ulcer (46.2%) and duodenal ulcer (42.6%), or between benign and malignant gastric ulcer (47.2 and 42.1%, respectively). Most of the fungi were yeasts of the genus Candida and Torulopsis. The mycel phase was demonstrable only in the native or histological preparation (16.7% [native] and 10.6% [histological] positive findings). In all those cases in which pseudomycels were demonstrated the culture count was especially high (more than 20 colonies). In most of these there was an additional disease which may be associated with immunological insufficiency (e.g. diabetes, chronic alcoholism, malignoma). The presence of pseudomycelia or more than 20 colonies in the culture from gastroduodenal ulcers are thus less of a pointer as to whether the local lesion is benign or malignant than to a general weakening of the organism's defence mechanisms.