Abstract. Let C denote the Fermat curve over Q of prime exponent l. The Jacobian Jac(C) of C splits over Q as the product of Jacobians Jac(Ck), 1 ≤ k ≤ l−2, where Ck are curves obtained as quotients of C by certain subgroups of automorphisms of C. It is well known that Jac(Ck) is the power of an absolutely simple abelian variety Bk with complex multiplication. We call degenerate those pairs (l,...