The highly crystalline alpha chitin occurring in the grasping spines of Sagitta was investigated by electron diffraction together with high-resolution electron microscopy. These techniques confirmed the presence of diffraction spots 001 and 0k0 with 1 or k of odd order. These reflections, which should be absent in the P2(1)2(1)2(1) space group, were identified as originating fi om double diffraction effects resulting from the high perfection of the samples as well as their thicknesses. This study confirms therefore the antiparallelism of the chitin chains in a system that can be considered as a model for or chitin. (C) 1995 Academic Press, Inc