THE interpretation of the structure of the spermatozoa of cestodes and of the process of spermatogenesis is subject to wide divergence of opinion. In 1935 Young1 directed attention to the lack of knowledge regarding cestode structure and development, and among the main problems awaiting solution discussed the method of cell division and the structure of the sperm. Regarding the latter he wrote that “we have even less information than we have about most cestode tissues and until the spermatogenesis is known this ignorance will continue”. His observations2,3 and those of Child4 “suggest a very simplified structure correlated with rapidity of development and the production of enormous numbers of spermatozoa”1. Watson5 believed that the spermatozoon of Gyrocotyle is provided with a head. He did not, however, make a detailed study of spermatogenesis and described the sperm as “a slender thread tapering at the posterior end, with a well marked head, several times the diameter of the body”. In a description of a figure of two spermatozoa he mentioned head, tail and acrosome. Presumably the acrosome is a short threadlike structure figured at the anterior end of the head. Since the publication of Young's paper1 no further information on the spermatogenesis of cestodes has become available.