The anthelmintic drugs mebendazole (Vermox®) and levamisole were tested in patients for their possible ovicidal effects on the eggs of Trichuris trichiura and Ascaris lumbricoides. The patients studied were 20 Africans of Sierra Leone, and 11 children of Nassau, Bahamas. Egg counts were made before treatment, and for 5 consecutive days following treatment. Eggs were incubated in 2.5% aqueous potassium dichromate and percentages of larval development were determined. Mebendazole completely inhibited the development of the eggs of T. trichiura by day 3 of the follow-up period. The ovicidal action of mebendazole on the eggs of A. lumbricoides was determined to be 64.8% by day 5 of the follow-up study. Levamisole had little (11.6%) ovicidal action on the eggs of T. trichiura and no ovicidal activity for A. lumbricoides.