In order to elucidate the association between the distribution and maturation of adult walleye pollock Theragra chalcogramma, otter trawl samplings were carried out in the Japan Sea off the southwest Hokkaido in October from 1989 to 1994. CPUE values of walleye pollock were significantly higher in the coastal region than those in the offshore region through the sampling years except for 1994. Proportions of males were significantly higher in the coastal region except for 1993 and 1994. Many males were nearly mature, and no differences in male gonad indices were found either between coastal and offshore regions or among the coastal areas. In contrast, females collected in the coastal region were more mature than those in the offshore region. Clear geographical differences in body length were not found either among males or among females. Sexual differences in reproductive behavior and maturity might be responsible for the skewed distribution pattern of adult walleye pollock in October.