Let (Mm,g) be a
compact Riemannian manifold isometrically immersed in a simply connected
space form (euclidean space, sphere or hyperbolic space). The purpose of
this paper is to give optimal upper bounds for the first nonzero eigenvalue
of the Laplacian of (Mm,g) in terms of r-th mean curvatures and scalar
curvature. As consequences, we obtain some rigidity results. In particular, we
prove that if (Mn,g) is a compact hypersurface of positive scalar curvature
immersed in ℝn+1 and if g is a Yamabe metric, then (Mn,g) is a standard
sphere.