The spermatozoon of the African catfish Clarias gariepinus is a simple organized aquasperm although it reveals very unique characteristics: the cytoplasmic channel is lacking, the mitochondria form a complex structure and the arrangement of the centriolar complex is species specific. Semen has high initial motility rates (c.70-90%) and swimming velocities (c.120-140μm s - 1 ), the main swimming type is linear. Motility duration in water is 30s and is prolonged only to 40s in NaCl solutions or more complex buffered motility activating saline solutions. A pH between 7.0 and 9.0 has no effect on the sperm motility parameters. Motility is completely and reversibly suppressed in electrolyte and non-electrolyte solutions with an osmolality of 200mosmol kg - 1 . During immotile storage the sperm viability is influenced by the osmolality and the potassium levels of the storage medium, by the temperature and by the dilution. At optimal conditions (buffered sperm motility inhibiting saline solution: 150mmol l - 1 NaCl, 2.5mmol l - 1 KCl, 1mmol l - 1 CaCl 2 , 1mmol l - 1 MgSO 4 , 20mmol l - 1 Tris solution, pH 8.5; dilution rate 1: 5; storage temperature, 4 o C) sperm viability persists for >7 days. High viscosity of the pure seminal vesicle secretion completely inhibits the sperm motility. When the seminal vesicle secretion is diluted in water the viscosity decreases and the motility suppressing effect is neutralized. When semen is mixed with seminal vesicle secretion the sperm viability decreases to zero within 10min.