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