In six villages near Isfahan in central Iran 1,455 persons were examined for intestinal parasites. Those with Ascaris infection were treated with pyrantel pamoate in a single dose of 10 mg/kg body weight and all stools passed during 48 hours after treatment were collected in plastic pans and screened for worms which were then sexed and measured. Ascaris infection rates, 87–95% in the six villages before treatment, were reduced to 1–8% (average 5%) and the mean number of eggs in the feces was reduced from 19/mg to <1/mg. All age groups and both sexes were about equally infected, and the average number of worms expelled by treatment ranged from 16 per infected person below 5 years of age to 31 per person 20–39 years of age. Mature and immature worms together were expelled from persons treated at all seasons, indicating that worms were acquired and lost continuously throughout the year. Fecal examination at 2, 4, 6, and 12 months after treatment showed that the prevalence at 12 months had returned to the original level (87% vs. 91%) but the average intensity as reflected in egg-counts had not (10 vs. 19/mg feces). The findings confirm the necessity of repeated treatments at 2- to 3-month intervals.