During 1982-1990, a longitudinal observation on prevalence trend of malayan filariasis has been made in endemic areas with An. anthropophagus as the main vector. A total of 22,795 person-times of blood examination were made, and 30,439 An. anthropophagus and 10,061 An. sinensis were dissected respectively. The microfilaraemia rate dropped from 1.0% to 0.14%, and the infection rate of An. anthropophagus decreased from 0.74% to 0.09% in 3 endemic villages, while no positive case or infected vector occurred in 5 villages where microfilaraemia cases were absent since the beginning of the study. In a cross-sectional survey, 855 villages of 17 counties has been monitored for 10 years. Out of 213,934 person-times of blood examination, only 56 were positive, the average microfilaraemia rate being 0.0262%, and 94.64% of the positive had already been detected before 1986. Based on these data, it has been suggested that in endemic areas with An. anthropophagus as main vector, when the microfilaraemia rate dropped to less than 1% after control, there was no indication that the rate would upgrade during the survey period. A decline trend of the transmission of malayan filariasis, therefore, has been exhibited.