Abstract: The invention concerns a method and apparatus for separating fluids containing particulate matter into two or more fractions of different densities. A cyclone separator (1) is utilized for this purpose and differs from previous constructions inasmuch as it is a uni-directional, air-cored device having an inlet (7) only for the fluid (A) in the top and two separate outlets (11, 15) in the bottom, remote from the inlet, one (11) of which outlets extend axially of the separator. In operation particles (as defined in the specification) once separated from the streamline (A') in which they enter the device are constrained against traversing that streamline. Means (5) are provided adjacent the inlet (7) to stabilize the initial flow of fluid. The axially extending outlet (11) may be adjustable.