There is no doubt about Aédes aegypti being a chief vector of the dengue viruses in Bangkok which are known as a virus group to cause Thai hemorrhagic fever (Rudnick and Hammon 1961, SEATO Ann. Frog. Res. 1964). In order to control the disease it is, needless to say, first to elucidate the ecology of the vector in Thailand. In the past, there were many papers on the ecology of A. aegypti in regard to yellow fever (Whitman 1951, Christophers 1960). For example, Durham and Myers (1902) described that A. aegypti is very active in the early afternoon but after sundown none, and this conception was supported by Gordon and Young (1921) using marked female of the mosquitoes. However, the result obtained by Bishop and Gilchrist (1946) indicates that the mosquito feeds readily on the chicken in dark and Lumsden (1957) reported that maximal catches in the but interior for females were notable at 8 a. m., 2 p. m., 9 p. m. and 3 a. m. in tropical Africa Tanganyika though the but interiors were practically dark even by day. Nevertheless A. aegypti is said to be in the main of active diurnal habit and is widely known in the tropics as the day-biting mosquitoes. The authors attempted to elucidate the diurnal rhythmus of the mosquito collected hourly from sunrise to sunset in the house in Bangkok keeping in mind any relation with the age of mosquitoes. The virus isolation from those collected mosquito was also attempted. The results so far obtained are presented briefly as a preliminary report.