In this paper the evolution of the sperm tail is discussed. The primitive motile apparatus is assumed to be a conventional '9 + 2' axoneme which persists in all aquatic phyla having external fertilisation. Where internal fertilisation has evolved in association with terrestrial life, the sperm tail has a '9 + 9 + 2' pattern: it has acquired new accessory proteins and has become enormously elongated. A subsequent trend is towards diminished motility, owing perhaps to the excessive development of skeletal structures and sophisticated copulatory organs. This is marked by unusual axoneme patterns and a lack of dynein arms. Aflagellate spermatozoa seem to represent a high evolutionary level. Finally, it appears that in the sperm of some groups motility has been regained. However, the axoneme never reappears: motility is produced instead by the spermatid manchette or an actin system.