In this paper, we find a new relation among codimension 2 algebraic cycles in M 1,4 . The main application of the new relation is to the calculation of elliptic Gromov-Witten invariants. For example, we show that if V has no primitive cohomology in degrees above 2, the elliptic Gromov-Witten invariants are determined by the elliptic Gromov-Witten invariants, together with the rational Gromov-Witten invariants.In [12], we will prove, using mixed Hodge theory, that the cycles [M(G)], as G ranges over all stable graphs of genus 1 and valence n, span the even-dimensional homology of M 1,n , and that the new relation, together with those already known in genus 0, generate all relations among these cycles. This result is the analogue, in genus 1, of a theorem of Keel [18] in genus 0.Our new relation is closely related to a relation in A 2 (M 3 ) ⊗ Q discovered by Faber (Lemma 4.4 of [8]); the image of his relation in H 4 (M 3 , Q) under the cycle map is the same as the push-forward of our relation under the map M 1,4 → M 3 obtained by contracting the 4 tails pairwise. This suggests that our new relation should actually be a rational equivalence. 1 Let us illustrate our results with the case of the projective plane. The genus 0 and genus 1 potentials of CP 2 equalwhere t 0 , t 1 and t 2 are formal variables, of degree −2, 0 and 2 respectively, dual to the classes 1 ∈ H 0 (CP 2 , Q), ω ∈ H 2 (CP 2 , Q) and ω 2 ∈ H 4 (CP 2 , Q) respectively, and N (0) n and N (1) n are the number of rational, respectively elliptic, plane curves of degree n which meet 3n − 1, respectively 3n, generic points. Kontsevich and ManinReceived by the editors February 10, 1997 and, in revised form, June 4, 1997. 1991 Mathematics Subject Classification. Primary 14H10, 14H52, 14N10, 81T40, 81T60. Key words and phrases. Gromov-Witten invariants, moduli spaces, algebraic curves. 1 Since this paper was written, Pandharipande [26] has found a direct geometric proof of the relation of Theorem 1.8, showing that it is a linear equivalence, by means of an auxiliary moduli space of admissible covers of CP 1 .