A transient grating method is used to excite and monitor Lamb acoustic waveguide modes in free standing thin polyimide films which are known to have transverse isotropic symmetry. The dispersion of these Lamb modes is used to accurately characterize both the in and out-of-plane elastic moduli in the films. To our knowledge, this is the first example of the noncontact, nondestructive determination of transverse isotropic moduli in a thin film system.