Either tembusu or umbre virus or both were recovered from three species of the Culex (Culex) vishnui subgroup of mosquitoes: Cx. vishnui, Cx. tritaeniorhynchus and Cx. pseudovishnui, which were collected in and near a chicken farm at the Dusun Tua study site in Malaysia. Cx vishnui, the most abundant species collected, was more successfully attracted to light traps augmented by CO2 than to any of the other five collection methods used. Ten out of 12 tembusu and four out of six umbre virus strains were isolated from Cx. vishnui, the majority of which were recovered during June, the peak month for mosquito collections. Tembusu virus activity appeared in late April and continued throughout July, whereas umbre was active in June and July. Tembusu and umbre viruses appear to utilize the same hosts, Cx. vishnui subgroup mosquitoes and domestic fowl in their transmission cycles.