Little information is available on growth rates and reproductive effort in microchiropteran bats that breed in temperate areas, are not colonial, and do not hibernate. We measured growth in individual young of the hoary bat, Lasiurus cinereus, a solitary, foliage-roosting, migratory species, and assessed growth rate using changes in forearm length. We tested the prediction that growth is slower in this than in other species because of the less stable thermal environment that adults and juveniles experience. Forearm length and mass of 1-day-old young (X̄ ± SE) were 19.11 ± 0.30 mm and 4.73 ± 0.20 g, respectively. Over 3 years, growth rate of young differed, with young growing slowest (1.14 mm/day) during the coldest year and fastest (1.45 mm/day) during the warmest year. Young were not weaned until 7 weeks of age and nearly 3 weeks after fledging and continued to gain mass over winter. Unlike other species, lactating females did not lose mass through the breeding season. Based on a Levenberg-Marquardt algorithm for nonlinear regression, the growth constant of young hoary bats (0.083 in females) is less than that documented for most other species breeding in temperate North America. Migratory habits of L. cinereus allow adults and young of the year to forage throughout winter and may be associated with slow growth in this species and production of relatively large litters in species of Lasiurus in general.