The lugworm,Arenicola marinaL. has an annual cycle of reproduction with epidemic spawning and external fertilisation. The spermatozoa ofArenicolaare unusual in that they are held immotile (as plates of several hundred cells known as morulae) in the coelomic fluid until activated just prior to spawning. Activation ofArenicolasperm is brought about by a sperm maturation factor (SMF) from the prostomium and can be carried outin vitrousing an assay technique developed by Bentley (1985). Scanning electron microscopy is used here to examine the changes which occur duringin vitroactivation. This revealed that the bundles of flagella of inactive sperm become disorganised as flagella beating commences but the flagella at this stage are still bound together at their tips. The sperm heads then become separated from the cytophore and finally the distal binding of the flagella is broken to give free-swimming spermatozoa. Coelomocytes present in the coelomic fluid resorb unspawned gametes prior to the initiation of the next gametogenic phase.