Repeated cross-sectional surveys among infants sleeping under insecticide-treated bed nets (ITBN) and contemporary control infants were used to estimate changes in Plasmodium falciparum exposure due to ITBN use on the Kenyan coast. Presence of P. falciparum parasites or total P. falciparum Immunoglobulin M (IgM) seropositivity were used independently and in combination in a constant risk catalytic conversion model to estimate the force of infection in ITBN and control communities. Such studies during infancy avoid problems of early saturation of prevalence due to high forces of infection and persistence of infection, minimize problems of self-treatment, and can be conducted among large populations covering a wide geographic area. These contrast previous parasitologic studies of ITBN among older children and the traditional entomologic studies of transmission that are logistically demanding. Our investigations demonstrated that parasite prevalence, IgM seropositivity, and the force of transmission were all significantly reduced by 50%. In addition, more infants under ITBN entered their second year of life without previous exposure to P. falciparum than control infants. These effects upon delayed acquisition of effective immunity require careful monitoring during future vector control programs using ITBN.