Let N be a prime and let A be a quotient of J 0 (N ) over Q associated to a newform such that the special L-value of A (at s = 1) is non-zero. Suppose that the algebraic part of the special L-value of A is divisible by an odd prime q such that q does not divide the numerator of N −1 12 . Then the Birch and Swinnerton-Dyer conjecture predicts that the q-adic valuations of the algebraic part of the special L-value of A and of the order of the Shafarevich-Tate group are both positive even numbers. Under a certain mod q non-vanishing hypothesis on special L-values of twists of A, we show that the q-adic valuations of the algebraic part of the special L-value of A and of the Birch and Swinnerton-Dyer conjectural order of the Shafarevich-Tate group of A are both positive even numbers. We also give a formula for the algebraic part of the special L-value of A over quadratic imaginary fields K in terms of the free abelian group on isomorphism classes of supersingular elliptic curves in characteristic N (equivalently, over conjugacy classes of maximal orders in the definite quaternion algebra over Q ramified at N and ∞) which shows that this algebraic part is a perfect square up to powers of the prime two and of primes dividing the discriminant of K. Finally, for an optimal elliptic curve E, we give a formula for the special L-value of the twist E −D of E by a negative fundamental discriminant −D, which shows that this special L-value * I would like to modify the title and include the words "and special L-values of twists". I have included the earlier title in case it was used as an identifier for the paper.We also call the ratio L(A/F,1) Ω(A/F ) , the algebraic part of the special L-value of A f over F ; in the contexts where we shall use this, it is known that the ratio is a rational number (and hence an algebraic number).If N is a positive integer, then let X 0 (N ) denote the modular curve over Q associated to Γ 0 (N ), and let J 0 (N ) be its Jacobian. Let T denote the subring of endomorphisms of J 0 (N ) generated by the Hecke operators (usually denoted T ℓ for ℓ ∤ N and U p for p | N ). If f is a newform of weight 2 on Γ 0 (N ), then let I f = Ann T f and let A f denote the quotient abelian variety J 0 (N )/I f J 0 (N ) over Q. We also denote by L(f, s) the L-function associated to f and by L(A f , s) the L-function associated to A f . It is known thatΩ(A f ) is a rational number. Now fix a newform f of weight 2 on Γ 0 (N ) such that L(A f , 1) = 0. Then by [KL89], A f (Q) has rank zero and X(A f ) is finite. Thus the second part of the Birch and Swinnerton-Dyer conjecture becomes:Conjecture 1.2 (Birch and Swinnerton-Dyer).