A study of the morphology of the genitalia and alimen- tary organs of Urostylidae shows that Urostylidae present a combination of features indicating a relationship to several pentatomomorphan taxa, notably Pyrrhocoridae, Tessaratomidae and Aradidae. It appears that Urostylidae and Pyrrhocoridae diverged from a Pentatomoid-Pyr- rhocoroid stock at an early date in the evolution of Pentatomomorpha, possibly close to the point where the Aradoidea branched off. to Southern and Eastern Asia where it reaches the Palearctic Region and the Malayan Archipelago. Records of its occurrence on the Australian mainland are believed to be incorrect (Yang, 1939), but a species occurs in the southern part of Papua. Although groups re- stricted in geographical distribution are often interesting as relict forms or as having special adaptations, the comparative morphology of the Urostylidae has received surprisingly little attention. Most speculations on their relationships are based on the study of single species. On the basis of study of the unexpanded aedeagus of a lone species, Pruthi (1925) related them to Acanthosomatidae. Pender- grast (1957), from an examination of the spermatheca of a single species, emphasized the relationships of Urostylidae and Pyrrho- coridae, a view originally put forth by Yang (1939) on grounds of several external features. Southwood (1956) studied the eggs and Scudder (1959), the female terminalia, but neither was able to com- ment on the relationships of the Urostylidae. More recently, how- ever, on the basis of the alimentary organs of Heteroptera, Miyamoto (1961) has considered Urostylidae closer to Pentatomidae than to Acanthosomatidae or Dinidoridae. Cobben (1968), from the study of eggs, has stressed that the occurrence of common genital features in Urostylidae, Acanthosomatidae and Dinidoridae is a case of parallel- ism and does not indicate close relationship. Recently the author has had the rare opportunity of studying the internal male reproductive organs, expanded aedeagi and spermatheca of several species of Urostylidae; the results are presented here.