ZnS: Mn has been in-filled in photonic crystals of submicron polymer spheres. The effect of the photonic band gap on the photoluminescence (PL) properties of ZnS: Mn has been investigated. Because of the overlap of the transmission dip of the photonic crystal and the photoluminescence band of ZnS: Mn, both suppression and enhancement in the PL of the phosphor have been observed. A strong dependence of the fluorescence lifetime on the emission wavelength in the range of the stop band has been found. This strong dependence is believed to arise from the very low photon density of state within the stop band of the ZnS: Mn in-filled photonic crystal as result of a high dielectric contrast between ZnS: Mn and the polystyrene spheres.