Six biotite and two hornblende separates from the Hidden Bay pluton, Adak Island, yielded K-Ar dates that cluster about an average of 33.13 ± 0.41 m.y. B.P. This average is interpreted as the time of intrusion of the pluton and provides the first documentation of an early Oligocene magmatic event in the central Aleutian island arc. Biotite and hornblende from the nearby Kagalaska pluton on Adak Island yielded nearly concordant K-Ar dates that have an average of 13.15 ± 0.55 m.y. B.P., in good agreement with published dates of about 13 to 14 m.y. B.P.