A study relating to seasonal prevalence and host visitation ofMansonia mosquitoes in Trivandrum city was carried out between August 1984 and July 1985. The area chosen was the one which is a perennial breeding source forMansonia mosquitoes. OnlyMansonia uniformis andMansonia annulifera were prevalent in this area of which the former was found to be the predominant species. Decrease in the number ofMansonia annulifera from the survey conducted during the previous year may be due to the disappearance ofPistia stratiotes and its replacement bySalvinia auriculata. Rainfall and temperature are the two important environmental factors which influence the number ofMansonia mosquitoes visiting the host. The maximum number of adults ofMansonia uniformis was collected during February to June 1985. No swarm ofMansonia uniformis was encountered during the present study, however, males were found to wait and mate with the resting as well as blood fed females in the vicinity of the host. Both sexes ofMansonia mosquitoes were parasitized byArrenurus and limnesiid species of water mites. Maximum parasitization was seen during July 1985.