One female of _P. truei_ lost or killed all but one young of her litter; at about the same time, a _P. maniculatus_ and all but one of her young inexplicably died. Since the remaining young _maniculatus_, a male, was just weaned and was considered expendable, I placed him in the cage with the female _truei_ and her 33-day-old, male offspring. The reaction to the newcomer was unexpected. The female immediately covered the _P. maniculatus_ and her own young and prepared to defend them against me. Later, when the _P. maniculatus_ was disturbed, he had only to emit a squeak and the female _truei_ would run to cover and protect him. When the young male of _P. truei_ was 69 days old the female kept him out of the nest, but still kept the male _maniculatus_ in the nest with her. Although the female was somewhat antagonistic to her own young, she did not injure him, but only kept him out of the nest. The male _truei_ was left in the cage with his mother and the _P. maniculatus_ from September 23 to December 10. None of the mice had any apparent cuts on the ears or tail to indicate fighting. As much as seven months after the _P. maniculatus_ was introduced into the cage, the female _truei_ continued to cover him with her body whenever there was a disturbance. The male _maniculatus_ not only tolerated this attention, but ran under the female _truei_ when frightened. "Adoption" of young of another species has been reported for a number of animals, but, without further evidence, it is not possible to postulate that such adoptions occur between species of _Peromyscus_ in nature. Young males are tolerated by their mothers after weaning. One young male _maniculatus_ was left in the cage with his mother from the time of his birth in autumn until late February of the following year. A litter was born on February 24. A young male _P. truei_ was also left in the cage with his mother until he had acquired most of his postjuvenal pelage; the female and male usually sat together in the cage. Females of both species sometimes eat their young when the young die shortly after birth. One female of each species killed three of her four young, and ate their brains and viscera. In one of these cases, the female, of _P. maniculatus_, also died; the female of _P. truei_ was the same one that adopted the surviving _P. maniculatus_. The female _truei_ continued to nurse her one remaining young for at least several days after killing three of his litter mates. A reason for this cannibalism might have been that I had fed these mice for several weeks on a mixture of grains low in protein content. Inadequacy of this diet for nursing females may have caused them to become cannibalistic. The feed of all captives was changed to Purina Laboratory Chow after the young were killed. Transportation of Young