A study to compare effects of mass, targeted and selective chemotherapy with levamisole (Ketrax) as an action for the control of Ascaris lumbricoides was carried out in three communities in rural Oyo State, Nigeria. Selective treatment was applied in one village by treating the most heavily infected 20% of the inhabitants, targeted treatment in the second village involved children aged 2-15 years, while mass treatment was offered to all inhabitants excluding infants under 1 year and pregnant women in the third village. Recommended doses of levamisole were given in the villages, as described above, at 3-monthly intervals during a period of 1 year. Prevalence and intensity (epg) of A. lumbricoides infection were determined immediately before and 3 months after the period of intervention using a modified Kato-Katz technique. In the selective treatment village, no significant differences were found between the pre- and post-treatment egg counts (mean (+/- S.D.) epg 6776 +/- 10,791 versus 4259 +/- 10,909 respectively) of A. lumbricoides in the total population. In the targeted treatment village, significant differences were recorded in pre- and post-treatment egg counts for the total population (9057 +/- 15,797 versus 2579 +/- 6529) among the children alone (10,935 +/- 20,094 versus 992 +/- 3175) and among the untreated adults (7742 +/- 9782 versus 4561 +/- 8798). In the mass treatment village, significant differences in pre- and post-treatment egg count values were also recorded (11,907 +/- 17,220 versus 1489 +/- 5165). The intensity of Trichuris trichiura and hookworm infections among the villagers before and after intervention were not observed to have changed significantly regardless of selective, targeted or mass treatment.