  In this letter, we provide evidence for universality in the low-energy
expansion of tree-level string interactions. More precisely, in the
alpha'-expansion of tree-level scattering amplitudes, we conjecture that the
leading transcendental coefficient at each order in alpha' is universal for all
perturbative string theories. We have checked this universality up to seven
points and trace its origin to the ability to restructure the disk integrals of
open bosonic string into those of the superstring. The accompanying kinematic
functions have the same low-energy limit and do not introduce any
transcendental numbers in their alpha'-corrections. Universality in the
closed-string sector then follows from the KLT-relations.
