Defects in the structure of the middle piece of poultry spermatozoon were studied by the electron-microscopic method. The defects included loose jointing of the head and falgellum owing to the shallow invagination fossa of the nucleus, and structural and mainly shape alterations of the proximal centriole. These defects existed in spermatozoa with a normal shape of nucleus as well as in those with rounded or otherwise irregularly shaped nuclei.