Spectral and polarization sensitivities of blowfly R1-6 photoreceptor cells were measured by intracellular recordings in cells which differed in visual pigment content. The spectral sensitivity in the visible wavelength range can be quantitatively explained from the absorption spectrum of blowfly visual pigment when the waveguide properties of the rhabdomere are taken into account. The peak wavelengths of absorption and sensitivity spectrum are 490 and 487 nm respectively. At low visual pigment content a sensitizing pigment can enhance the relative U.V. sensitivity from 0.3 to 2.0. In flies with a high visual pigment content selfscreening substantially broadens the visible band of the spectral sensitivity and lowers the relative U.V. sensitivity. The gain of the electrical response appears to be independent of the visual pigment content.