A set S of vertices in a graph G is a semitotal dominating set of G if it is a dominating set of G and every vertex in S is within distance 2 of another vertex of S. The semitotal domination number is the minimum cardinality of a semitotal dominating set of G. We observe that the semitotal domination number of a graph G falls between its domination number and its total domination number. We provide a characterization of trees that have a unique minimum semitotal dominating set