The magnesium alloy AZ91 has been studied under different solidification conditions, to establish the details of the as-cast structure. The electron backscattered pattern (EBSP) technique has been used to determine the crystallographic directions of the dendrite stem and the secondary arms. Under unidirectional solidification conditions, two different stem directions were found: at a low temperature gradient and high growth velocity, the dendrites grew in a «11-20» direction, while at higher temperature gradients and lower growth velocities, the growth direction was (2245). Dendrites with «11-20» stems have arms in six directions around the stem, while dendrites with «22-45» stems have three secondary arm directions. The crystallographic directions of the secondary arms are the same as the two stem directions.