Abstract:Let s ≥ 2 be an integer and k > 12(s − 1) an integer. We give a necessary and sufficient condition for a graph G containing no K 2,s with δ(G) ≥ k/2 and (G) ≥ k to contain every tree T of order k + 1. We then show that every graph G with no K 2,s and average degree greater than k − 1 satisfies this condition, improving a result of Haxell, and verifying a special case of the Erdős-Sós conjecture, which states that every graph of average degree greater than k − 1 contains every tree of order k + 1.