A magnetostrictive actuator comprises a rod (2) of giant magnetostrictive material, a biasing permanent magnet (4) arranged at each end thereof and coaxially therewith so as to pass a magnetic field through the rod, and an energising electromagnetic coil (6) connected to energising means so as to superimpose on the magnetic field passing through the rod an alternating magnetic field to cause variation in the length of the rod in response thereto, wherein the permanent magnets (4) are each in the form of a disc having a cross-sectional area substantially greater than that of the rod and are spaced from the ends of the rods by material (3) of low magnetic permeability, such that the magnetic field in the rod is substantially linear along the length thereof.
