In this paper we give an explicit
construction of the formal group of the Jacobian of an algebraic curve using a basis
for the holomorphic differentials on the curve at a rational non-Weierstrass point. We
construct the formal group of the Jacobian of the modular curve X0(l) and using a
result of T. Honda, we prove that this formal group is p-integral for all but finitely
many p.