In this divorce action the husband was awarded custody of the three children born during the marriage. Evidence established the husband is not the biological father of two of the children. Because of this, the husband is a third party in relation to them and may be awarded custody only upon a finding that the natural mother is unfit. Childs v. Childs, 237 Ga. 177 (227 SE2d 49) (1976). The trial court did not reach the issue of the mother’s fitness, so we reverse and re*208mand for consideration of this issue.

Judgment reversed and remanded.