At the age of 4 months, a boy in Isfahan, Iran, was examined because of an illness, of several weeks, characterized by irritability, cough, diarrhea, sweating, and pallor. After each exacerbation of his illness, he had expelled four worms, which proved to be Moniliformis moniliformis. During the next 5 months, five M. moniliformis were expelled; all were male worms. Repeated stool examinations revealed no helminths or helminth eggs, but Giardia lamblia was found on one occasion. The child had moderate anemia and leukocytosis, with 4% eosinophils. Fourteen days after treatment with thiabendazole, the ninth worm was expelled, and on the 15th day eggs of M. moniliformis were found in a stool specimen. A rat captured in the patient's home was infected with a female M. moniliformis containing eggs. Cockroaches and a lizard caught in the home were not infected with this or other parasites. We believe that a grain beetle in barley, eaten by the child, may have been the intermediate host.