Special values of the modular j function at imaginary quadratic points in the upper half-plane are known as singular moduli; these are algebraic integers that play many roles in number theory. Zagier proved that the traces (and more generally, the Hecke traces) of singular moduli are described by a multiply infinite family of weight 3/2 weakly holomorphic modular forms of level 4 (or, through what is sometimes called 'duality', by a multiply infinite family of weight 1/2 weakly holomorphic modular forms of level 4). Several authors have used this description to obtain relations and congruences for these traces modulo prime powers p n in various situations. We prove that the modular forms in question satisfy a simple relationship involving the Hecke operators T (p 2n ) for n 1. As a corollary we obtain uniform relations for the traces (some of which were known in particular cases).Each J m can be expressed as a polynomial in j; for example, we have J 0 (τ ) = 1, (τ ) + 159 768 = q −2 + 42 987 520q + . . . , J 3 (τ ) = j(τ ) 3 − 2232j(τ ) 2 + 1 069 956j(τ ) − 36 866 976 = q −3 + 2 592 899 910q + 12 756 069 900 288q 2 + 9 529 320 689 550 144q 3 + . . . . Alternatively, we may define J m := J|T 0 (m), where T 0 (m) is the Hecke operator of index m and weight 0 (normalized so that its action preserves integrality). For each m 0 we define the mth Hecke trace of the singular moduli of discriminant −d as tr m (d) := Q∈Q d /Γ