The magnetic and transport properties of epitaxial and polycrystalline chromium dioxide (CrO2) thin films have been investigated. They are grown epitaxially on single crystal TiO2 (100) substrates, and with multiple grain orientations on polycrystalline TiO2 substrates, by chemical vapor deposition. The films have a Curie temperature (TC) of 390–395 K, with the epitaxially grown CrO2 (100) films exhibiting in-plane uniaxial magnetic anisotropy. While the epitaxial samples display metallic characteristics, the polycrystalline films are semiconducting with a dominant grain boundary contribution to the resistance at low temperatures. The magnetoresistance (MR) properties have also been measured with the magnetic field applied in the plane. For the epitaxial films, the MR is negative at temperatures near TC and is positive at low temperatures. A negative MR is also observed near TC for the polycrystalline samples. However, unlike the epitaxial films, the MR is found to be negative also at low temperatures, with a significant low field component. The latter is attributed to spin-polarized transport of electrons across grain boundaries. © 2000 American Institute of Physics.