In a further study of data from the literature and from personal observations on the seasonal and spatial distribution of Dermacentor variabilis (Say) [see preceding abstract], comparison of the weather records in south-western Wisconsin with the life-cycle of this tick in that area showed that temperature was the main governing factor. The first larval activity peak occurred at a mean temperature of 12.5 deg C in April-May, and the second (after 2 cold periods) at 18.5 deg C in late May; this confirmed previous records of a bimodal pattern of spring activity for Dermacentor larvae [see RAE/B 64, 2404] but not the attribution of it to the appearance of larvae from 2 different generations. Summer larvae from spring eggs were most numerous in mid-July over the whole of Wisconsin, and in a single locality with a mean temperature 2.6 deg C lower than the state mean the summer larval peaks occurred about 2 weeks later. Calculations are made from the state temperature records on the probable dates of the different development stages. The size of the larval cohorts was not always what would have been expected from a consideration of the effects of temperature alone on the parent generation, but this was attributed to resistance developing in the rodent hosts in the summer after previous exposure, a factor not usually taken into account as a regulating influence on populations of D. variabilis.