Observations of the external morphology of wild-caught and captive male brown long-eared bats (Plecotus auritus) revealed a marked seasonal pattern of spermatogenesis, similar to that established by histological examination of other temperate-zone vespertilionids, with a peak in testes size in August and a lag between testicular growth and epididymal distension. This was followed by complete cessation of spermatogenesis and shrinkage of the testes to prepubertal size. The period of peak spermatogenesis coincided with a reduction in the incidence of torpor and also a decline in body mass. This suggests that torpor and spermatogenesis may be incompatible in this species. Outside the season of testicular growth and epididymal distension, sexual maturity could not be confidently determined in P. auritus by the pigmentation of the tunica vaginalis surrounding the epididymis, a characteristic which has become established in other species. Instead, the size and shape of the caudae epididymides appeared to be a better criterion for defining sexual maturity in this species. Most males underwent testicular growth and epididymal distension, and were assumed to have reached sexual maturity, at an age of 15 months. However, some individuals (29%), particularly those in good condition (relatively heavy individuals), showed a degree of testicular and epididymal development in their first autumn, i.e. at 3 months of age, indicating that the onset of puberty may be dependent on body condition. Poor body condition was associated with delayed spermatogenesis in adult males.