Abstract Hole transport and optical properties were investigated on undoped and Mg-doped LaCuOS 1− x Se x ( x=0–1) epitaxial films. Both electrical conductivity and Hall mobility were found to increase monotonously with increasing Se content in the films. The increase in Hall mobility is considered to be associated with the increase in valence band dispersion. Mg ion doping increased hole concentrations in the undoped films by an order of magnitude to ∼2×10 20 cm −3, while Mg doping reduced mobility to merely half that of undoped films. The results suggest that hole scattering due to Mg impurity ions is suppressed by natural modulation doping originating from the layered structure of LaCuOS 1− x Se x . Hole concentrations showed no temperature dependence, indicating degenerate conduction. The largest value for conductivity, 140 S cm −1, was obtained with Mg-doped LaCuOSe epitaxial film. Accompanying characteristics included moderately high optical transparency in the visible region and blue photoluminescence.