The growth, development and general behavior of Nyctomys sumichrasti were observed during a 13-month period. Adults are primarily nocturnal. In the laboratory, family units or individuals raised together reside with each other; strangers to the group are not tolerated. Mating behavior, copulation and care of the young are described. The gestation period is approximately 30 days. Litter size averages two. The young reach adult size in approximately 6 weeks. Rate of development is rapid. The young crawl slowly at 2 days of age, and well at 5 days. When the eyes open at 15 to 18 days, coordination approximates that of the adults.