To estend the study on bird spermiogenesis we have observed by electron microscopy the mature testis of the cuculiform Crofopbaga ani.During nucleus elongation the chromatin condenses in dense granules and the nucleoplasm is reducing. The Golgi apparatus starts the developing of the acrosomal vesicle. The elongating nucleus drives the acrosomal vesicle against the cell membrane. At the opposite side the flagellum arises from the distal centriole and comes out from the cell at a very early stage. \Vhile the cytoplasm is reducing, the mitochondria arrange themselves around the distal centriole and around the fore part of the asoneme.The elongating spermatid does not have a microtubular manchette or a microtubular bundle, as seen in other birds. It is entirely surrounded by a Sertoli cell, which shows many microtubules. scattered in the cytoplasm. These microtubules are arranged parallel to the axis of elongating spermatids. Near the Sertoli cell membrane, surrounding the acrosome, is electron-dense material.The perforatorium, present in the sperm of Galliformes and Anseriformes, is absent. A subacrosornal space, found in Columbiformes sperm, is also absent.The morphogenetic factors influencing the head shape of this sperm seem to be peculiar and probably related to the microtubular apparatus of the Sertoli cell.