Abstract. The members of Martin-L¨of random closed sets under a distribution studied by Barmpalias et al. are exactly the infinite paths through Martin-L¨of random Galton-Watson trees with survival parameter 2 3 . To be such a member, a sufficient condition is to have effective Hausdorff dimension strictly greater than γ = log2 3 2 , and a necessary condition is to have effective Hausdorff dimension greater than or equal to γ.