We outline the differences between three notions of rate of increase: r, the observed rate of increase; rs, the rate implied by the prevailing schedules of survival and fecundity, and rm, the maximum rate at which a population with a stable age distribution can increase in a specifie,d environment. Several mammalogists recently calculated r8 for natural populations under the misapprehension that they were calculating rm. However, their calculated values are usually de£ective even as r8 essdmates because they have used life tables constructed from age distributions or from distributions of age at death. The estimates are thereby infiltrated by the unrecognized but implicit assumption that rs = O, and the calculated values of ss are therefore assumptions retrieved as conclusions. If rm, the intrinsic rate of increase, is to be detelmined for a natural population of mammals, it is best calculated either by measuring the rate at which a newly established population initially increases or by fitting a curve to the growth of a population after its density has been artificially reduced. Over about the last 10 years mammalogists have become increasingly interested in population dynamics, and they have begun using concepts and analyses hitherto restricted almost entirely to the demegraphy of man and inselets. This is a welcome trend that should be continued7 but at the same time a warning is necessary against uncritically using special methods of analysis that may not be appropriate to field studies of mammals. By this we do not imply that the demagraphy of mammals differs esslentially from that of insects; we suggest only that some parameters, wholse estimation is relevant to questions that entomalogists ask) may be extremely difficult to estimate for natural populations of mammals. In addition, the paramleters may be irrelevant to the problem in hand. The reverse is also true. Mammalogists ask questions on conservation and harvesting, for example, that entomalogists seldom consider, and they must therefore estimate parameters that entomologists ignore. In this paper we examine some of the ways in which rate of increase can be measured and point out that some mammalolgists, using equations that are valid and widely used in insect ecology, are estimating it incorrectly. 658 KINDS OF RATE OF INCREASE Because populations tend to grow geometrically, rate of increase is best expressed in expl(}nential form. A population of 100 animals ffiat increases to 200 over a year has been multiplied by 2 or increased by 100 percent, however o!ne prefers to express it. Its exponential rate of increase, r, is given by