Adult female pipistrelle bats were captured from two colonies in eastern England before the time of parturition. Bats from each colony were fed freely in the laboratory. Growth models were used to estimate growth rate and asymptotic size for each neonate. The proportion of bats giving birth was greatest in those from Colony 1 (P< 0·03). The logistic growth model gave a better description of juvenile growth than either the Gompertz or the von Bertalanffy models. Bats from Colony 1 had slower growth rates than bats from Colony 2 (P<0·005). Gravid females from Colony 2 were heavier on the day after birth than those from Colony 1 (P< 0·005). Bats from Colony 1 gave birth earlier than those from Colony 2 (P<0·001) but weaned their young later than Colony 2 (P< 0·001). Males were born earlier than females in the sample from Colony 1 (P< 0·001). Growth of males and females was similar. These results show that post-natal growth rates of pipistrelles are highly variable and can approach the growth rate of birds. Growth rates of young bats are probably also affected by environmental conditions experienced by mothers during gestation.