The twinning activity in random textured cast magnesium during monotonic, room temperature tension and compression tests was monitored by neutron diraction. Decrease of integrated intensity which characterizes the twinned volume fraction of selected reections was compared to its Schmid factor. The comparison shows that twinned fraction correlates with the maximum value of the Schmid factor with high precision during tensile test and with the average value of the Schmid factor during compression test.