The spermatozoa ofBoops boops, Diplodus sargus, Mullus barbatus, andTrachurus mediterraneuswere motile in sea water, and in electrolyte solutions (NaCl) and non-electrolyte solutions (glucose) with an osmolality of 600-1000mosmol kg - 1 . Their mean motility rate 10s after initiation was about 80%, while about 10% of the motile spermatozoa moved non-linearly, 45% linearly, and 45% circularly. The average path swimming velocity was significantly higher inM. barbatus(about 90μm s - 1 ) than in the other species (70μm s - 1 ). The number of motile spermatozoa decreased to 0% within 50s after initiation of motility inT. mediterraneus, within 90s inM. barbatus. InB. boopsandD. sargusabout 90% of the spermatozoa stopped movement during the first 90s of the motility period, while the rest remained motile for 2-3h. Motility ofB. boopsandD. sargusspermatozoa was reversibly suppressed in the seminal plasma, and in electrolyte and non-electrolyte solutions of 100-200mosmol kg - 1 . The trigger for motility activation was hyperosmolality (700-1000mosmol kg - 1 ). Motility ofM. barbatusandT. mediterraneussperm was only partly suppressed in the seminal plasma since freshly collected semen contained about 25-50% locally motile spermatozoa. When sperm was activated immediately after collection with electrolyte and non-electrolyte solutions of 700-1000mosmol kg - 1 spermatozoa moved progressively. The motility of those spermatozoa which had not yet been motile after collection was completely and reversibly suppressed inM. barbatusat osmolalities of 1200mosmol kg - 1 , and at osmolalities of 100-200mosmol kg - 1 inT. mediterraneus. Therefore two triggers were necessary for initiation of motility. The nature of the first trigger was uncertain, the second trigger was a switch to hypoosmolality inM. barbatusand to hyperosmolality inT. mediterraneus. The sperm organisation ofB. boops, D. sargus, M. barbatusandT. mediterraneusrevealed species-specific parameters which could not be related with the sperm motility behaviour.