This study investigated whether sexual imprinting plays a role in the recognition of the sex of conspecifics. Subjects were zebra finch males that had been raised with either normal pairs, white pairs or pairs of both morphs. They were tested for their preferences in six two-stimuli tests covering all combinations of both morphs of either sex. Males of all groups showed a clear preference for females of their mother's morph over males of their father's morph. Moreover, a majority of the males from mixed parentage preferred a male of their mother's morph over a female of their father's morph, indicating that morphological characteristics are more important than behavioural differences for discrimination between the sexes. This was confirmed in a subsequent series of tests in which males raised by parents of the white morph, which are sexually dimorphic only in respect to the intensity of the red colour of the bill, were given a choice between a white male with his bill painted orange and a white female with her bill painted red. All males of this group preferred to court the male in this test. In addition to differential responses in a sexual context, males showed discrimination in an aggressive context; that is, most aggression was directed towards stimulus birds resembling the father. The implications of these results for the evolution of sexual dimorphism and sexual selection are considered.