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The population identity of humpback whales Megaptera novaeangliae in the Arabian Sea has long been a matter of dispute New information is presented from this region, based upon whaling and obsel-vat~ons conducted by the Sov~et Union, primarily in November 1966. In that month, a total of 238 humpbacks were killed off the coasts of Oman, Pak~stan and northwestern India; 4 others were killed in 1965. Biological exannnat~on of these whales showed that they differed s~gn~flcantly from Antarctic humpbacks in terms of size, coloration, body scars and pathology. In a d d i t ~o n , analysis of the length distribution of 38 foetuses indicates that the reproductive cycle of the Arabian Sea whales was unequivocally that of a northern hemisphere population. Mean lengths were 12.8 m for males (range: 9.5 to 14.9 m, n = 126) and 13.3 m for females (range: 9.5 to 15.2 m, n = 112). All whales 12.5 m or more in length were sexually mature. Among 97 females examined. 12 (12.4":21 were immature. Of the 85 mature females, 39 (45.9%)) were pregnant, 3 (3.5%) were lactating, and 43 (50.6%) were resting. A more plaus~ble pregnancy rate, adjusted for underrepresentation of lactating females, was estimated at 39%). A malority of stomachs examined contained food, including euphausiids and fish. Overall, the data presented here argue strongly that Arabian Sea humpbacks constitute a discrete population which remains in tropical waters year-round, a situation which is unique for this specles.