SEM was used to study the external morphology of spermatozoa from two species ofMacrobiotus(Eutardigrada). In both species, the spermatozoon consists of 4 parts: (1) a rod-shaped acrosome; (2) a helical nucleus; (3) an elongated, swollen middle piece, with lateral vesicles; (4) a tail ending in a tuft of thin and long filaments (probably microtubules). Observations madein vivoand on fresh preparations of sperm cellstotoconfirm the SEM images. Moreover, light microscopy findings also showed that spermatozoa in the testes and gonoducts have a folded-back acrosomal-nuclear region, joined to the middle piece. Artificially extracted spermatozoa appeared in some cases straight-headed.