The motility of the spermatozoa of freshwater fish is usually of short duration, but a precise description has rarely been provided. Motility requires a high dilution (more than 1,000-fold) for initiation of synchronous motility in 100% of spermatozoa; a two-step procedure is necessary, with an initial dilution of 1 to 100 in a medium that keeps the spermatozoa immotile and allows good mixing of the viscous semen. The second dilution (1 to 20) in the activating solution can be made directly under the microscope. Studies of carp sperm indicate that movement of live sperm is influenced by the ionic environment. The inhibition of motility in semen is mainly due to K+ ions in trout and osmotic pressure in carp, but other ions such as Na+, H+, and Mg2+ also interfere. Initiation of motility in trout requires external divalent cations. Immediately after dilution at 20°C, spermatozoa exhibit large circular trajectories (>400 μm in diameter), high beat frequencies (60 Hz), and velocities of 250 μm/sec. These values decrease rapidly. Within 20 sec after dilution, most spermatozoa stop moving, although some of them show some agitation with low beat frequency (< 10 Hz) and with very limited displacement during the next few minutes. A similar pattern is observed in carp, with active motility lasting 40 sec. Under certain ionic conditions, intratesticular spermatozoa are motile and have some fertilizing capacity.