The structure and development of the spermatozoon of the nematode Heterakis gallinamm has been described. The spermatozoon is amoeboid. The centrioles have the unusual structure of nine single fibres. The nucleus has no limiting membrane and is surrounded by mitochondria and organelles, here called alpha bodies. These alpha bodies appear to arise from Golgi complexes in association with granular endoplasmic reticulum in the spermatocytes and have a fibrillar component which is released into the peripheral cytoplasm of the spermatid when the spermatozoon is formed. There is no refringent cone as in other ascaroid spermatozoa.