We use the Jacquet-Langlands correspondence to generalize well-known congruence results of Mazur on Fourier coefficients and L-values of elliptic modular forms for prime level in weight 2 both to nonsquare level and to Hilbert modular forms.A celebrated result of Mazur says that, for N a prime and p a prime dividing (the numerator of) N −1 12 , there exists a cusp form f ∈ S 2 (N ) = S 2 (Γ 0 (N )) congruent to the Eisenstein series E 2,N of weight 2 and level N mod p [Maz77, II(5.12)]. Further, if p = 2, one has a congruence for the algebraic part of the centralwhere η K is the quadratic character associated to a quadratic field K/Q [Maz79]. For instance, if N = 11 and K is not split at 11, there is one cusp form f ∈ S 2 (N ) and one gets that L alg (1, f K ) ≡ 0 mod 5 if and only if 5 ∤ h K . (If K splits at 11, the root number is −1 so L(1, f K ) = 0). A form of Mazur's L-value result was reproved by Gross [Gro87] for K/Q imaginary quadratic using quaternion algebras and the height pairing, whereas Mazur used modular symbols. Ramakrishnan pointed out to me that one can also deduce this from his average L-value formula with Michel [MR12]. (Note Gross's argument also involves an averaging type procedure, so these two arguments are not entirely different in spirit.)In this article, we use the Jacquet-Langlands correspondence and an explicit L-value formula to extend these results of Mazur both to more general levels and to parallel weight 2 Hilbert modular forms over a totally real field F with K/F a quadratic CM extension. For simplicity, we only state our results precisely for F = Q in this introduction.We first discuss the Hecke eigenvalue congruence result and a nonvanishing L-value result, and will state the more precise result on L-value congruences below in Theorem B. Theorem A. Let N be a nonsquare, and write N = N 1 N 2 where (N 1 , N 2 ) = 1 and N 1 has an odd number of prime factors, all of which occur to odd exponents. Let p be a prime dividing (the numerator of ) 1 12 ϕ(N 1 )N 2 q|N 2 (1 + q −1 ) and p a prime of Q above p. Then