Effect of a 0.2-T static magnetic field on the microstructure of a direct chill cast Al-9.8wt%Zn alloy slab was investigated. The static magnetic field transferred the microstructure from a mixture of equiaxed and columnar grains with the primary trunks growing in < 100 > directions to twinned lamellar feathery grains with the primary and secondary arms growing in < 110 > directions. The application of the static magnetic field results in the reduction of the heat discharge and solute mixing capacity through a damping effect on convection and thus a delay of the melt transformation to solid and a request to reduce the liquid/solid interface energy through reducing the interface area due to the loss of undercooling. The delay and the request account for the growth direction change and the formation of lamellas. The difference between the Al and Zn atomic radii and the related incoming flow facilitate the formation of the twins.