Summary.The simultaneous swimming up and down from identical semen specimens suggested that the separation efficiency of progressively motile sperm by the swim‐down method was superior to that of the swim‐up method. The swimming down was performed in a small plastic funnel, the bottom of which was plugged with a 2.5 ml disposable syringe. In this funnel, 2.0 ml of 70% Percoll was poured and then the sperm resuspension was overlaid. After 1 h, progressively motile sperm penetrating into the lower part of Percoll layer (1.6 ml) was collected in the syringe. The technical strategies to yield higher density of progressively motile sperm were found to be centrifugal concentration of the sperm from whole ejaculate prior to the swimming down, and subsequent re‐centrifugation of the separated sperm after the swimming down. Oligo‐asthenozoospermic semen (22 ± 4.4 × 106ml 5.5 ± 4.4% motillity,n= 8) was processed by the present method, yielding sperm qualities of 32 ± 19 × 106ml−1, 74 ± 14% in the final preparations. Overall improvement in fertility index (sperm density ml−1× motility % × 10−8) reache