^{Optical monitoring of diffusivity in wide bandgap semiconductors was performed by using a picosecond light-induced transient grating technique. The bandgap renormalization and carrier-carrier scattering manifested itself at room temperature as two-fold decrease of the ambipolar diffusion coefficient Da in cubic SiC and 5-fold decrease of Da in diamond at excess carrier density N > 1017cm-3, while for GaN the impact was observed only at T 19}cm^-3, the plasma degeneracy led to enhanced D_avalues in SiC and GaN and compensated the diffusivity decrease in diamond.