Spermiogenesis and mature spermatozoa ofMaehrenthaliasp. (Byrsophlebidae) andBresslauilla relicta(Graffillidae) were investigated by transmission electron microscopy. Spermiogenesis inMaehrenthaliaclosely resembles the process previously observed in Temnocephalida, some dalyelliids and other typhloplanids. Dense heels form at the base of basal bodies and these later become spur-shaped in the mature sperm, and rotation of flagella around the shaft results in compression of a semi-circle of peripheral microtubules in towards the centre of the shaft. The inner microtubules join the outer row over a short length of the shaft and do not form a spiral arrangement as is found in temnocephalids. Mature sperm have two free flagella that appear to be oriented distally, away from the main shaft, at least while sperm are in the ducts. Spermiogenesis inB. relictainvolves fusion of two initially free flagella, first with each other and then with the spermatid shaft, and attachment zones are associated with each axoneme. Incorporated axonemes are superficially attached externally to the cortical microtubules. In both species investigated, longitudinal rows of small granules lie beneath the cortical microtubules in mature sperm, there are two kinds of dense bodies, one or two rows of mitochondria, and the nucleus extends a trailer to the plasma membrane. It is now apparent that there is a useful range of character states observable in spermiogenesis and mature sperm of rhabdocoel turbellarians that will eventually facilitate parsimony analysis to determine the phylogeny of this large, diverse taxon.