After a large introduction concerning the influence of sperm quality in the success of in vitro fertilization, the debated problem of the importance of the sperm quality in intracytoplasmic sperm injection is investigated. The spermatozoa are studied by electron microscopy, and the results evaluated by the formula of Baccetti et al. (1995). The quality of spermatozoa has been correlated with the success of ICSI. The first conclusion has been that ejaculates which produced embryos contained higher percentages and overall higher total number of 'healthy' spermatozoa than those which obtained fertilizations arrested at the two pronuclei stage, or showed no fecundation at all. A second conclusion was that the quality of sperm organelles is mainly involved in the oocyte activation, but after the 2 pronuclei stage, segmentation usually proceeds with few influences from sperm quality. The highest involvement of spermatozoa is therefore the initiation of oocyte activation. A third conclusion confirmed the need of ultrastructural evaluation of sperm cell organization, deeply investigating the inner organelles. Moreover, we observed that the head organelles are more involved that the tail ones. Three acrosomal characteristics are concerned: the shape, the dimensions and the content. All of them are significantly better in the oocyte activating than in non activating spermatozoa. Also the nuclear shape and the status of the chromatin (frequently strictly interdependent) are significantly concerned with the oocyte activation and with the embryo segmentation. In the tail, the mitochondrial shape is significantly different in activating and not activating spermatozoa, and also in those producing embryo segmentation. Moreover, the absence of dynein arms seems to be determinant in impeding the oocyte activation.