We have studied the effect of surface roughness on the behavior of surface polaritons in samples of GaP by means of Raman scattering. The roughness of the surfaces of thin-slab samples (\ensuremath{\approx} 15 \ensuremath{\mu}m thickness) was controlled by the size of the final polishing powder and by etching and annealing. The surface-polariton linewidth broadens as the surface is roughened, and the dispersion curve is depressed below the theoretical value based on the flat-surface assumption. Both the increase in damping and the shift in frequency are well accounted for by existing theories. The most striking effect of surface roughness is the increase in the Raman-scattering intensity for which there is no adequate theoretical explanation.