 Head lice infestation occurs frequently, primarily in children 3 to 11 years of age. In recent years an increasing prevalence of lice infestation in schools, day care centres, and summer day camps is believed to be partly due to increasing lice resistance to currently available standard treatments for pediculosis infestation with lice. There is a need for new effective backup treatments for this common condition of head lice infestation. This is a double-blind, randomized, parallel-group, study in several clinical centres comparing ivermectin as tablets to malathion 0.5 lotion in the treatment of head lice. All enrolled patients participate in the primary phase of the study up to the Day 15 evaluation. Patients who are still infested with lice at Day 15 treatment failures will enter an extension phase and be treated in a double-blind, fashion with the opposite treatment ie ivermectin or malathion. The study hypothesis is that ivermectin will be more effective than malathion in clearing head lice infestation , as measured by the proportion of patients who are lice-free at Study Day 15. 