We propose to carry out a large field trial in The Gambia, West Africa, to find out whether there is any additional benefit of spraying homes with DDT in addition to the normal practice of getting everyone to sleep under bednets impregnated with long-lasting insecticides. The study will have two groups of villages: one with only treated nets and the other with both treated nets and DDT spraying. We will measure how effective the interventions are by sampling mosquitoes in the houses and measuring malaria in study children in both sets of villages. Importantly we will determine whether the vectors are likely to become resistant to the insecticides used and we will be able to determine how much it will cost to prevent cases of malaria using the DDT. This study is important since many Afircan countries, including The Gambia, are either spraying houses with DDT at present or intend to do so in the near future.