Seabass Dicentrarchus labrax sperm concentration was high (up to 60 10 9 spz ml 1 ) but decreased significantly at the end of the reproductive season (mid-March) in monthly sampled fish. The spermiation period may be shortened by frequent stripping. Sperm can be prediluted up to 1 : 128 in non-activating medium without loss of initial motility and motility duration. Immediately after activation by transfer to sea water, all the spermatozoa were motile for 10 s and then the number of motile cells decreased progressively but sharply to zero, so that the duration of sperm motility was very short (40 s). As a consequence, the fertility of seabass sperm decreased exponentially after 10 s following sperm activation and was zero by 1 min. The sperm requirements for optimal fertilization were c. 66 000 spermatozoa per egg. Scalingup of the experimental insemination procedure yielded better fertilization rates while conserving the individual differences due to the breeder pairs. 1999 The Fisheries Society of the British Isles