Two-loop scalar diagrams from string theory

We show how to obtain correctly normalized expressions for the Feynman diagrams of $\Phi^3$ theory with an internal $U(N)$ symmetry group, starting from tachyon amplitudes of the open bosonic string, and suitably performing the zero--slope limit by giving an arbitrary mass $m$ to the tachyon. In particular we present explicit results for the two--loop amplitudes of $\Phi^3$ theory, in preparation for the more interesting case of the multiloop amplitudes of non--abelian gauge theories.