The author reported a case of gastric myiasis, in which numerous first-instar larvae of Parasarcophaga crassipalpis were found on the ulcerous lesion of cancer in the operatively removed stomach. The larvae were all. revealed survived on the mucous membrane of the ulcerous lesion. The fact suggests that the larvae might be involved primarily in developing the ulcerous lesion or secondarily in making grow worse the stomach lesion.