The external morphology of spermatozoa from 14 gastrotrich species belonging to the order Macrodasyida was examinedin vivoby SEM and light microscopy. In all species the spermatozoa are filiform and consist of two or three regions. The anterior region is a spiral, but its distal and proximal segments are often differently coiled. The central region is not coiled, is of varying length or may even be absent. Intemally it consists of either a crystalline mass or the nucleus surrounding a mitochondrial axis. The posterior region is the tail. The ultra-structural data confirm the similarity of the basic structural plan of the spermatozoa in the macrodasyids. However, the individual genera of the same family, as well as different species of the same genus, present particular features. Thus, sperm morphology is a useful taxonomic criterion at both generic and specific level.