The authors studied the monthly and annual variation of hook rate (catch per 100 hooks) of yellowfin tuna in the tropical western Indian Ocean. From the study, the following results are obtained. 1) Good fishing in sub-area A could not show, in sub-area B it was seen in November to May and in sub-area C it was seen in October to May. 2) Since the starting of the exploitation by Japanese long-line fishery, the hook rate has declined in each sub-area, though temporary increase was noted around 1960; i.e. sub-area A in 1959 and 1960, sub-area B in 1960 and 1961, and 1959 in sub-area C.