Epidemiological survey on prevalence of Plasmodium falciparum anti-circumsporozoite antibodies (Ab-CS) was carried out in 21 villages of Comoros FIR, at the beginning of 1988 rainy season. Evaluation of anopheline indoor resting densities was also carried out at the same time. Frequencies of antibody-positive subjects vary considerably in the different villages, according to the Anopheles gambiae and A. funestus densities per room, which are determined by ecological factors. Ab-CS prevalence varies in the sample population from 5.5% in 3-4 years children to 40% in those of 5 years. Starting from 6-7 years group, prevalence increased steadily reaching a plateau by 30 years of age. The detection of Ab-CS levels in a sample population is a good tool to evaluate malaria transmission levels, especially where the epidemiological situation does not allow a reliable entomological evaluation.