The results of laboratory studies on nine species of warehouse Ptinid beetles at 25° C. and 70%r.h. are used to calculate by Leslie's methods, the statisticrmeasuring the intrinsic rate of increase of each species.Ptinus tectusBoield. has the highest value ofr—0–38, equivalent to a 39 % weekly increase or tenfold in under 2 months. Four species,Gibbium psylloides(Czemp.),Mezium affineBoield.,Stethomezium squamosumHinton andTrigonogenius globulusSol., are capable of tenfold increase in 3 months and the remaining four multiply more slowly, either because of a low oviposition rate (Eurostus), high developmental mortality (Niptus), or slow development (Ptinus fur(L.),P. sexpunctatusPanz.).Generally the intrinsic rate of increase falls with temperature due to lengthening of development and reduction of oviposition, but near the maximum of development it also falls because of high developmental mortality.Niptushas a higher rate of increase at 20° C. than at 25° C. which is close to its developmental maximum. Relative humidity has little effect except at the limits of humidity tolerance, but water must be occasionally available for the adult to drink, or oviposition and thus rate of increase is markedly depressed. Food mainly affects the rate of increase through its effect on developmental speed. Other factors are briefly considered.The statisticrapplies to a population which has a stable age distribution. For Ptinids the stable age population was found by calculation to contain between 5 and 11% of adults, whereas in the warehouse never less than 15% of the population found were adults. In a warehouse actual observed rates of increase reached a value equivalent tor= 0.24 per week over a period of 8 weeks. From observed temperature means and laboratory results a maximum ofr= 0.31 was predicted with a mean of 0.19 per week for a year. The value ofrobtained using the oviposition rate estimated in the warehouse was 0.1. Warehouse values ofrmay be below those predicted from laboratory results because oviposition is restricted due to lack of drinking water and often to overcrowding, mortality increased due to disease, parasites, predators, and accidental cannibalism, and developmental speed retarded due to cr