Simultaneous and independent measurements have been made of the magnitude and direction of the lattice twist and of the direction of the Burgers vector of the axial dislocations in filamentary crystals of AlN, using electron microscopy and electron diffraction. The twist was revealed by extinction contours from reflecting planes with normals perpendicular to the filament axis, and measurements of the widths and separations of these contours were used to calculate the twist.Good agreement with theory was found in the case of crystals containing the simple axial dislocation, but crystals with a complicated extended axial imperfection were found not to be twisted. It is believed that the extended axial imperfection was a fault which met the surface of the crystal and which originated from an axial dislocation; the lack of twist was then due to one or more partial dislocations having left the crystal.