A system for separating motile and non-motile sperm comprises a microvolume into which a fluid containing sperm is delivered, at least partly defined by a wall which includes a termination or a change in angle away from the microvolume, defining an exit from the microvolume. Sperm delivered into the microvolume in a fluid that are motile move entrained along the wall and leave the wall at the termination with an outward turn to or towards a collection reservoir or passage from the microvolume. Typically the system is implemented on a microfluidic chip.
