Data for 1938-42 on the seasonal activity of Dermacentor variabilis (Say) and on the mean winter temperatures at Gay Head in Massachusetts were compared in order to test the theory that unusually high winter temperatures, by reducing water stress, increased adult survival and consequently the level of adult activity in the following spring [see RAE/B 64, 2409]. Although the activity level found in 1938 did not show the expected increase following a mild winter, more frequent (weekly) collections made in 1939-42 indicated the expected rise when average winter temperatures were above 0 deg C, the greatest increase occurring in 1939 after a winter with all mean monthly temperatures above 0 deg C and monthly maxima above 2 deg C. Comparison of similar data from the adjacent coastal area of Cape Cod for 1969-81 confirmed the theory and indicated the means by which low temperatures reduced adult survival; on bare soil, the repeated freezing and thawing of the surface that occurred at mean temperatures below 0 deg C resulted in the maintenance of a desiccated soil surface unfavourable to adult survival.