Abstract: The study was carried out in Muai Township of Junlian County, Sichuan Province, from July 12 to September 10, 1984. Among 1746 febrile patients, amounting to 22.9% of the total population (7619), 648 had vivax malaria. The incidence was 8.51%. The parasite rate and the gametocyte rate of the inhabitants were 19.1% and 4.46% respectively. Among 19456 anophelines examined, the predominant species were A. lesteri anthropophagus, A; sinensis and A. minimus. The percentages of A. lesterl anthropophagus in the human dwellings in the daytime and night were 79.4% and 69.7% respectively; and those of A. sinensis were 18.7% and 29.8% respectively. Howe ver, A. sinensis accounted for more than 90% of all the anophelines caught in cattle sheds, and A. lesteri anthropophagus only less than 5%. A. minimus was scarce both in human dwellings and cattle sheds. The man-biting rates of A. lesteri anthropophagus and A. sinensis were 0.91 and 0.41 respectively. The human blood index was 0.825 for the former and 0.053 for the latter. Among 12757 A. lesteri anthropophagus dissected, 47 were found to be gland positive, the sporozoite rate was 0.37%. No positive mosquito was found in 2 965 A. sinensis or 262 A. minimus dissected. The vectorial capacity of A. lesteri anthropophagus was 0.654 while that of A. sinensis was 0.019.The entomological inoculation rate (he) of A. lesteri anthropophagus calculated by the product of man-biting rate and sporozoite rate was 0.003 367; while that of A. sinensis was 0.000 185 (sporozoite rate was calculated according to MacDonald's formula). According to Krafsur's (1978) formula (R= l - e-het), the probability of inoculation of sporozoite from A. lesteri anthropophagus to the local inhabitants within the 60 days was 0.1829, while that from A. sinensis 0.0110, i. e. 94.3% of the local malaria cases were likely transmitted by A. lesteri anthropophagus, while 5.7% were by A. sinensis.The results suggest that A. lesteri anthropophagus is a vector of high efficiency and its potential in malaria transmission might be 20 times that of A. sinensis.