During the period from April 1971 to March 1972,a study was made on the seasonal change of composition rate of autogenous Culex pipiens molestus (Cpm) and anautogenous C.p. pallens (Cpp) in an underground room of a building in the midtown area of Nagoya City, central Japan. (1) For differentiating Cpm females from Cpp, the following five characters were used : Ommatidial number of the compound eye (square value of average number per row : x_1) and the following ratios of the various parts of wing, namely, Rc/Mc ratio (x_2), WL/Rs ratio (x_3), P/D ratio (x_4) and Rc/Rs ratio (x_5). On the bases of the discriminant values calculated using the discriminant function, Z_5=x_1-1.207 x_2+0.1953 x_3-28.99 x_4-3.322 x_5,mosquitoes with the smaller discriminant values than the critical point (25.24) were regarded as Cpm and those with the larger values as Cpp. As a result, most of the discriminant values were less than the critical point and the modes of them were within the range of 16.1 to 22.0; this suggested that most of the female mosquitoes collected with a light trap in the building seemed to be Cpm (Fig. 1). (2) A small number of these specimens, however, were considered to be Cpp, judging from their larger discriminant values. These Cpp mosquitoes were obtained mainly during spring to autumn, when their field populations usually prevailed in this district. Monthly rates of Cpp females were highest (ca. 10%) in midsummer, but the annual rate was as low as about 2% (9/480) (Fig. 1 and Table 1). (3) The number of female Cpm mosquitoes collected in the building, as compared with that of male, was predominantly larger in the monthly catch during the summer. So that, the monthly intermixing rate of Cpp was seemingly low in females, but considerably higher in males during the summer season (Table 2 and Fig. 2). (4) A confirmatory test was made as follows in order to put the discriminat function to the practical use. The above-mentioned function (Z_5) was used only for the indistinguishable specimens (0.8%) which had the transformed ommatidial numbers between 67.52 (8.22/row=1% lower rejection limit for Cpp) and 73.30 (8.56/row=1% upper rejection limit for Cpm). The result thus reached was proved to be the same as that obtained from the calculation for all the specimens under study.