Study Design: Balombo area (Angola) f from each other to avoid the possibility of active flights of the mosquitoes and “contamination” of treated or control villages. Methodology: Classical Cross Sectional Surveys (CSS) were symptomless children <15 years. During data analysis this sample was stratified into the 3 conventional age groups: <5 years (often considered as “at risk group"); 2-9 years (often used for classical endemicity index) and <15 years already used in other vector control studies. The 3 classical parasitological indicators: plasmodial prevalence, parasite load, gametocyte prevalence were compared between these 3 age-groups to select the most relevant for further analysis and evaluation of the efficacy of vector control implemented. Blood thick films prepared in the field were colored and microscopically examined at the Malaria Control Program's laboratory of the medical department of the Angolan oil company Sonamet (Lobito). Results: 4625 thick drops were made during the 38 regularly carried out field surveys. Plasmodium falciparum was the predominant species with few mixed P. falciparum + P. malariae infection and a single P. malariae one. The overall Plasmodic index was 42.7%, gametocyte index was 4.4% and high parasite load (> 10.000 par./ml) were noticed in children, even very young, without clinical symptoms. Classical seasonal variations of plasmodic index and some variations according to age group were observed. Gametocytic indices showed relatively stable levels with age group. Conclusion: Statistical analysis showed that “under 15 years” age group could be a relevant indicator to evaluate the efficacy of a vector control programme and it increases the sample size allowing to perceive even small variations induced by vector control. Variability of parasitological index according to age groups, villages, season, confirmed the importance of regularly surveys to know precisely the situation before the implementation of control operations for reliable further evaluation. Missing such base line data induced the failure of a former vector control project in Angola.