Growth of Peromyscus maniculatus borealis, P. m. maniculatus, and P. leucopus was examined in relation to season and temperature. Average weight of P. m. borealis and P. m. maniculatus at weaning was positively correlated with average minimum temperature at that time. Growth from weaning to 40 days of age did not differ seasonally or among populations, although young P. m. borealis and P. m. maniculatus were relatively smaller and larger, respectively, than young P. leucopus. Growth after 40 days of age varied seasonally. Young mice that had the potential to mature during the summer of their birth continued to grow after 40 days of age. Young mice that could not mature during the summer of their birth stopped growing when 75–80% of adult size was reached.