The bounds of variation for definition of a species or its separation into infra-specific taxa have long presented a problem to the taxonomist. The need for discretion is this regard, based on experience and observation, is still present. The possible applications of biochemical analyses to systematic relationships and evolutionary patterns are as yet largely unexplored. The correlation of such data with that of the classical taxonomist should yield some insight into the problem. Further conclusions may be possible when work now in progress, including all of the known species of Proterometra, has been completed. The first application of these methods [chromatography] to the class Trematoda is the concern of this paper.