Abstract: Apparatus for mixing or otherwise agitating fluid material comprises an elongated vessel (1) to contain the material, means (6, 7, 9, 10) to impose oscillatory motion upon the material in a lengthwise direction, and a plurality of stationary obstacles (19, 35) mounted on the inner wall (2) of the vessel and arranged in sequence lengthwise. The obstacles present sharp ridge-form tips (20, 37) each ridge pointing in a direction (30) at right angles to that of the oscillating motion. Each adjacent pair of obstacles and the length of vessel wall between them define a trough-shaped space (26) in which the oscillating motion repeatedly forms vortices and then ejects those vortices vigorously into the remainder of the fluid outside the trough, so promoting the agitation of that fluid. The invention applies particularly to cylindrical vessels in which the obstacles are ring-shaped (19) or are constituted by the successive turns of a helix (35).