The first extended zeroth-order connectivity index of a graph G is defined as 0c1(G)=SvIV(G) D-1/2v where V(G) is the vertex set of G, and DV is the sum of degrees of neighbors of vertex v in G. We give a sharp lower bound for the first extended zeroth-order connectivity index of trees with given numbers of vertices and pendant vertices, and characterize the extremal trees. We also determine the n -vertex trees with the first three smallest first extended zeroth-order connectivity indices.