Application of the principles of population analysis to the genus Tetragoneuria Hagen has shown the value of this approaach, not only for determining taxonomic entities and testing the validity of their distinguishing characters, but also for obtaining evidence on the degree of relationship among taxa. In replacing the older type-species concept, the population-species concept of the new systematics presents numerous opportunities for objective re-evaluation of the classification of groups in which, as is true in many odonata, minor morphological differences have been virtually the sole basis for separating supposed entities. Tetragoneuria is such a group: the taxonomic characters which had been used were not only few but rather minor; several of its recognized forms, in consequence, have dubious status. In the study reported here, two subspecies and one species, previously thought distinct when studied by Muttkowski and others under the then prevailing typological concept, are synonymized. From the population viewpoint it is evidemt that these forms are merely variants, parts of species populations which exhibit clinal variation in size and coloration. Considered as populations, the three species Tetragoneuria cynosura, T. spinigera, and T. canis were found to be distinct as inmatures and adults on the basis of linear measurements; the measurements not only differ significantly in their means but also have significant degrees of nonoverlap of distribution curves for many characters. In addition, the various lines of evidence derived from the study of these species (as populations) have revealed degrees of relationships not previously evident. Of these relationships, those of primary interest are the closeness of that between cynosura and spinigera and the rather distant one that exists between those two and canis.