THE controversy among ecologists on ‘density dependence’ swelled out of all proportion due to differences among authors in the meaning and use of such words as ‘carrying capacity’, ‘competition’ and ‘density’. Beneath these unimportant disagreements, however, there lies at least one fundamental question which has seldom been investigated: Does an area have for a species of organism a carrying capacity with the property that the population decreases when it exceeds the carrying capacity and increases when it is less ? Few doubt that each area has such a carrying capacity in theory, but there is little evidence that the carrying capacity is ever reached or has much bearing on the actual size of population. Rather, as Andrewartha and Birch1 have claimed, it is often more useful to ask how recently the population met a catastrophe, to what level it was reduced, and at what rate it is able to recover toward the theoretical carrying capacity until it is overtaken by the next catastrophe. (Smith2, on the other hand, was able to detect evidence of a carrying capacity in Andrewartha's data, but even so it was rather poorly correlated with the actual population.) When extra vertebrates are stocked into an area their population quickly falls to the pre-stocked level3. Thus ecologists have long known that areas reached their carrying capacity in so far as vertebrate populations are concerned; the real question is whether the same is true with respect to invertebrates. It is the purpose of this communication to describe some experiments which not only demonstrate the existence of a carrying capacity for the checkerspot, Melitaea harrisii, but also to show, in general, what determines this carrying-capacity.