In mid-March 1981 33 bluebird boxes were erected 3 to 5 km south of Sonoita, Santa Cruz Co., Arizona, in oak-juniper grassland habitat at approximately 1540 m. On 11 May I found four just-hatched Mountain Bluebirds (Sialia currucoides) and one egg. Both parents were carefully observed at close range, appeared in excellent physical condition, and showed no signs of hybridization with either Eastern (S. sialis) or Western (S. mexicana) bluebirds; both species occur in the area, the Eastern commonly nearby and the Western in lesser numbers primarily in the Santa Rita Mountains, about 20 km west, and the Huachuca Mountains, about 25 km east. On 26 May I returned with Max C. Thompson who confirmed the identification and assisted in banding five well-feathered young which immediately left the box and hopped and fluttered toward a nearby tree. We put them back, apparently successfully, as none was seen to exit and both adults brought food for the next half hour. Neither the young nor the parents were seen again but no specific search was made for them. Eastern Bluebirds occupied the box in June and produced three young in July. Gale Monson (in litt.) confirmed this as the southernmost nesting record of the Mountain Bluebird, whose nearest known nests in Arizona are approximately 240 km to the north. It winters regularly in the Sonoita area in small flocks.