The growth and development of 16 young from six litters of the kangaroo rat, Dipodomys stephensi , are described in detail from birth to 63 days. This is the first detailed study of growth and development for kangaroo rats adapted to semiarid rather than arid habitats. Increase in dimensions is analyzed through measurements of absolute size, percent of adult size, and instantaneous growth rates at various ages. For D. stephensi , the percentages of adult dimensions attained by 63 days of age are: tail, 86.7%; body, 90.3%; hind foot, 96.3%; ear, 96.1%; vibrissae, 92%; cheek pouch, near 100%; weight, 70.1%. Growth data for D. stephensi are compared briefly with previously published data for D. merriami and D. deserti , two species highly adapted to the extremely hot, arid southwestern deserts. In general, growth rates of stephensi are intermediate between those of deserti and merriami , with the last the most precocious in development. The practice of recognizing straight-line phases of development within which the instantaneous growth rate remains constant is discussed, with the conclusion that growth rates are more conveniently described by smooth curves constructed from instantaneous growth-rate values computed for short-time intervals.