Magnetic domain structurea in thick (rw 1 mm) single-orgstal slices of yttrium iron garnet have been observed by a magneto-optic technique using transmitted polarized infrared radiation. The structures revealed that the crystals had a uniaxial or orthorhombic anisotropy that had been induced during growth. haw of the induced anisotropy, the bulk domein structures were markedly different from surface colloid pattern. Domain structures which are chracteristic of the intrinsio magnetocrystalline anieotropy are observed in dices in which the induced anisotropy has bean removed by a heat treatment.