Summary Mosquito-borne diseases, other than malaria, are becoming important public health problems in Bangladesh. Knowledge of the bionomics of the vector species is a pre-requisite for planning effective control measures against insect-borne diseases. With the above end in view, the bionomics of the common mosquitoes of Dacca city have been discussed under four headings, viz., seasonal prevalence, host preference, activity periods, and probable relation to diseases. Twenty-seven species of mosquitoes belonging to 6 genera were collected during the work. Sixteen of these were few in number and no inference could be drawn about their bionomics. Culex pipiens fatigans is a dry weather species; it has the maximum population between November and March. C. tritaeniorhynchus and C. gelidus are abundant during the wet months; the former is most abundant in July and the latter in September. The Anopheles, Aedes, and Mansonia species showed two population peaks, one before the monsoon (March–April) and the other during the monsoon (August–October). The pre-monsoon peak tends to be higher than the monsoon peak. Anopheles vagus, An. subpictus, and An. hyrcanus var. nigerrimus had the peaks in March and August; An. annularis had them in March and October. Mansonia annulifera and M. uniformis had the peaks of population in April and August. Aedes aegypti and Ae. albopictus had them both in April and September. Anopheles vagus, An. hyrcanus, and An. subpictus are highly zoophilic. The first two of these species did not bite man during this work; a few An. subpictus bit man at day time. Anopheles annularis, Mansonia annulifera, M. uniformis, Culex gelidus, and C. tritaeniorhynchus showed zoophily; they were more attracted to cow than man during this work. Culex pipiens fatigans and Aedes aegypti showed slight anthropophily. Twenty species of resting mosquitoes were collected at night compared to only 14 species at day time. Number of mosquitoes collected per man-hour at night was approximately 50% higher than that collected at day time. Although Culex tritaeniorhynchus feed mainly at night yet a considerable number of them fed at day time also. Aedes aegypti and Ae. albopictus were more active during the day time. In Dacca city Culex pipiens fatigans, C. tritaeniorhynchus, Aedes aegypti, and Ae. albopictus are likely to be the important species in the transmission of various mosquito-borne epidemic diseases.