Sexually mature zebrafinch males show a sexual preference for the species which has raised them. IMMELMANN (1972 a, b) stated that such a preference develops more readily if zebrafinch males were exposed to their own species than to Bengalese finches. The explanation for this phenomenon has been the supposition that an initial bias for the own species exists which is independent of post hatch experience. This hypothesis presupposes the absence of differences in behaviour between zebrafinch and Bengalese finch parents raising zebrafinch offspring. To investigate this supposition, the parental behaviour of zebrafinch and Bengalese finch (foster)parents raising zebrafinch offspring was observed. This was done in pure pairs (male and female of the same species) as well as in mixed pairs (male and female of different species). Differences between both species were found in parental care, and in clumping, allopreening, aggressive and sexual behaviour directed to zebrafinch young; most clearly in mixed, but often in pure pairs too. These differences were present from a few days after hatching till the moment of separating parents and young at about day 57. The kind of differences gradually changed during this period. Zebrafinch young are thus not exposed to equivalent experience with zebrafinch and Bengalese finch parents. The observed differences in experience may underly the more readily developing preference for zebrafinch than for Bengalese finch. This alternative should be explored more thorougly before the initial bias hypothesis is accepted.