Daia us plexippus (Linne) and Danauis gilippus berenice (Cramer), the Monarch and Queen butterflies, overlap broadly in their ecological requirements (Brower 1961a). Further evidence of their similarity was indicated by comparative laboratory analyses of egg cannibalism (Brower 1961b), developmental rates, and food consumption (Brower in prep.). Briefly summarized, these investigations showed that in south central Florida the adult butterflies of both are highly attracted to the same flowers from which they obtain nectar and also oviposit on the same plants which are shared as food by their larvae. Moreover, their larvae are highly prone to egg cannibalism and the 2 species eat each other's eggs to approximately the same extent as their own, although the larvae of the Queen are nearly twice as cannibalistic as those of the Monarch. Their rates of development from egg to adult differ by less than 10%, but the Monarch requires more than one anad a half times as much larval food as the Queen to complete its development. Indirect evidence that the species compete in nature was suggested by the fact that there is a periodic change between extensive sympatry and nearly complete allopatry as the aninual migration cycle of the Monarch occurs. Nevertheless, before almost completely vacating the southeastern wintering grounds as summer advances, the Monarch breeds extensively in the same areas as the Queen. That they are actively competing was suggested by the fact that the reduction in numbers of the Monarch coinci(le(l with a build-up of the Queen population, and that durini, the heavy spring breeding of the Monarch, the Queen females for the most part avoided the most common species of Asclepias foodplant but later readily oviposited on it after most of the Monarchs had left (Brower 1961a). This paper will present further evidence of competition based on additional field studies of their breeding populations in south central Florida during the summer of 1960.