Female brown long-eared bats Plecotus auritus nurse their own young selectively. Choice experiments suggested that females recognized infants by using both olfactory and acoustic cues. Infant isolation calls (i-calls) showed individual variation, and vocal signatures allowed the females to recognize and suckle their own pup. As the pups grew, the i-calls increased in frequency and decreased in duration, and the i-calls appeared to change into orientation cries. Mothers were able to recognize recent calls of their own babies in preference to older calls. Echolocation sounds were distinguishable statistically between lactating females, and the behaviour of the babies during choice experiments suggested that in this species a mutual acoustic recognition occurs. The growth and development of wing shape in P. auritus are described.