Equilibrium humidities of the larvae of 3 medically important tick species, the Rocky Mountain wood tick, Dermacentor andersoni (Stiles), the American dog tick, Dermacentor variabilis (Say), and the Cayenne tick, Amblyomma cajennense (Fabricius), were determined. Equilibrium weights can be maintained in the larvae of all 3 species at humidities down to approximately 80% to 85% relative humidity. Survival experiments with larvae of D. variabilis and A. cajennense demonstrate that the longevity is abruptly shortened at humidities below the equilibrium humidity.