The electron microscopical analysis of spermatozoa in two infertile male carriers of a pericentric inversion in one of the chromosomes 9 revealed the presence of a peculiar defect affecting the tails' fibrous sheath in both patients. This structure appeared completely disorganized and hyperplasic; sometimes the defect was associated with other usual malformations concerning the nuclear and acrosomal shape and texture and the axonemal assembly. Most spermatozoa (90-100%) of these patients were immotile. Our findings point to a definite ultrastructural sperm defect found in cases of autosomal inversion.