Observations on the behavioral development of a male timber wolf, hand-reared from the age of four weeks, have been continued for a period of three years. Data include the following: (1) Adjustment to human companionship has been successful even after sexual maturity. (2) Behavior towards strange persons and objects is cautious. (3) The animal remains more independent and aloof to persons than similarly reared dogs. (4) Interaction with dogs, including males and young puppies, is marked by gregariousness and lack of aggression. (5) Small domestic species have been attacked occasionally, the response depending largely upon the behavior of these animals. (6) Large species are avoided, chased, or ignored. (7) Numerous motor patterns, postures, and communicative signals are closely similar to siblings and other wolves and contrast with dog companions. (8) Sharp behavioral changes occurred at eight and 12 weeks, the former initiating aggressive tendencies, the latter extreme avoidance in the presence of new objects. Restlessness, then calming, followed in later months, and slight seasonal fluctuations have been noted. (9) Rudiments of many adult motor patterns occurred at an early age, e.g., small prey-pounces and pelvic thrusts seen by eight weeks. (10) Vocalizations include a wide repertoire of howls, squeaks, growls, and barks.