We have used phase-sensitive ultrashort-pulse interferometry to study the modifi cation of optical

pulse propagation near the photonic band edges in colloidal crystals consisting of polystyrene spheres in

water. A strong suppression of the group velocity is found at frequencies near the L gap of the fcc lat-

tice. The group velocity dispersion diverges at the band edges and shows branches of both normal dis-

persion and anomalous dispersion, which can be interpreted as large changes in the effective mass, both

positive and negative. We obtain excellent agreement with the dynamical diffraction theory.