The present account deals with the results of direct observations on the humpback whales brought into the floating factory “Kyokuyô-maru” during the Antarctic season 1938/39. Dur-ing the period extending from 18th November 1938 to 8th February 1939, a total of 248 whales was examined and more than 70% of them were caught in the area between 61°-63°S. and 100°-105°E.<br> (1) Of 248 humpbacks examined 174 were females and 74 were males ; thus the sex-ratio of the whales showed great segregation of females in the above-mentioned area.<br> (2) The colour pattern of the humpbacks was divided by L_ILLIE'S method according to the gradation occurred in the amount of black and white as shown in Fig. 2. The colour type of the whales was rather alike to that of New Zealand group than that of South Africa and South Georgia groups. The differences in colour were not accompanied by any other distinguishable characters.<br> (3) Examination of the genitalia showed That female humpbacks become sexually mature at a length of about 12•1m. This length is somewhat shorter than that (12•5m) suggested by M_ATTHEWS (1937). The smallest mature female recorded was 10•9m. long while the largest immature one was measured 12•4m.<br> (4) The average number of corpora lutea of 89 females (the average length of which was measured 12•78m.) was 4•78. It was seen from this that female humpbacks were consisted of average same age during the 1924-1931 season at South Georgia and South Africa and during the 1938/39 season in the mentioned area.<br> (5) Of 16 adult females with only one corpus luteum 10 were pregnant, I was lactating and 1 was recently ovulated or pregnant. Thus at least 11 (69%) whales would be estimated as they were pregnant in the first breeding season and nearly the same results were obtained also in the whales with two or three corpora lutea.<br> It might be tentatively concluded same as blue whales that number of unfertilized ovula-tions or formations of corpora atretica before pregnancy, if any, must be very few. Neither multiple ovulations nor multiple pregnancies were observed throughout the season.<br> (6) The apparent constitution of the, stock i.e. the ratios of the different classes of whales examined might be summarized as follows. (See page 52).<br> High proportion of pregnant whales among mature whales were obvious against low pro-portion of resting and recently ovulated or pregnant whales when compared with blue and fin whales.<br> (7) Assuming the females caught belong to an independent stock, the stock of whales in the area were tentatively calculated (W_HEELER'S method on fin whales was adopted) not wi th standing insufficient data and heterogeneous composition of the catch.<br> Analysis of the age groups of mature females determined by the corpora lutea during the season showed a rate of reduction of 32% on an average and besides it was proved by calcu-lation that as there produced only about 64% of immature females necessary to keep the stock at its present level, considerable damage is being done in the area concerned.