The variability of four characters commonly used in the separation of species ofLimnodrilushas been described, and their relative values assessed. The number of setae per bundle is useless as a specific character; the shape of the setae can serve to distinguish only a single species and the length: breadth ratio of the penial sheath is more useful as an additional or confirmatory character. The shape of the penial sheath proved to be the most important character, but even this was not completely reliable, and it is concluded that several characters must be used in the identification of species ofLimnodrilus. It is suggested that structures other than the cuticular derivatives should be examined and that a similar study be carried out on North American species.