Colossal magnetoresistance in excess of 106% has been obtained (at 110 K,H=6 T) in epitaxially grown La–Ca–Mn–O thin films. The as‐deposited film exhibits a substantial magnetoresistance value of 39 000%, which is further improved by heat treatment. The magnetoresistance is found to be strongly dependent on film thickness, with the value reaching the maxima at ∼1000 A˚ thickness, and then reduced by orders of magnitude when the film is made thicker than ∼2000 A˚. This behavior is interpreted in terms of lattice strain in the La–Ca–Mn–O films. ©1995 American Institute of Physics.