Broods of young zebra finches were either raised by their own species or cross-fostered by Bengalese finches. Both were isolated visually from other birds at the age of 2 months. Sexual preferences of males were examined at maturity, in a choice between 1 zebra finch female and 1 Bengalese finch female, by measuring the number of song strophes directed at each. Males raised by their own species sang exclusively for the zebra finch female. Cross-fostered males raised without siblings or with only 1 sibling sand exclusively for the Bengalese finch female. A large number of cross-fostered males raised with 2-4 siblings showed at least 5% singing for the zebra finch female, and a small number did so predominantly. It is concluded that under the conditions of these experiments, the development of sexual preferences of zebra finch males is affected by both parental and sibling influences. It is possible that the earlier evidence for a predisposition to response to conspecifics put forward by Immelmann could be explained by hitherto uncontrolled differences in experience.