Experimental and theoretical investigations are made of the generation of vertical Bloch lines in a magnetic iron garnet film exposed to pulsed optical radiation. High-speed photography and anisotropic dark-field microscopy are used to study characteristic features of the generation of Bloch lines and domain structure relaxation processes after the local action of a laser pulse. Optimum optical irradiation parameters to ensure the controlled generation of Bloch lines are established. A theoretical model is developed which links the generation of Bloch lines to the migration of domain walls induced by local changes in the distribution of the degaussing fields caused by a reduction in magnetization with temperature at the optical radiation focusing point. The experimental results indicate that the controlled formation of magnetic structures smaller than or of the order of 0.1 μm by local optical irradiation is quite feasible.