When an intense laser pulse propagates through a cluster gas it is shown that a plasma channel with a refractive index higher than its surrounding region is produced. In the high intensity regime, that leads to a novel self-confinement effect which keeps the laser beam from diverging. This effect is of importance for ultrashort pulse durations, less than 100–300 fs, depending on ion mass, and is limited by the expansion process caused by strong heating in the near-solid density cluster microplasma. A computational study of the self-confinement effect has been performed where long plasma channels were demonstrated. ©1998 American Institute of Physics.