Let 911 be the mapping class group of a surface of genus g > 3, and 9 the subgroup of those classes acting trivially on homology. An infinite set of generators for 9, involving three conjugacy classes, was obtained by Powell. In this paper we improve Powell's result to show that 9 is generated by a single conjugacy class and that [911, 9] = 5. I. Let M = M , be an orientable surface of genus g ...