Abstract. Let (A, a) be an indecomposable principally polarized abelian threefold defined over a field k ⊂ C. Using a certain geometric Siegel modular form χ 18 on the corresponding moduli space, we prove that (A, a) is a Jacobian over k if and only if χ 18 (A, a) is a square over k. This answers a question of J.-P. Serre. Then, via a natural isomorphism between invariants of ternary quartics and Teichmüller modular forms of genus 3, we obtain a simple proof of Klein formula, which asserts that χ 18 (Jac C, j) is equal to the square of the discriminant of C.