Studies were conducted to determine the feasibility of marking Dermacentor variabilis (Say) with the alkali metal rubidium (Rb) by feeding larvae on mice that were injected with rubidium chloride (RbCl). Newly molted nymphs that fed as larvae for 2 to 5 d on RbCl-injected mice contained approximately 660 parts per billion (ppb) of Rb, whereas nymphs that had fed for 6 to 10 d as larvae on RbCl-injected mice contained 419 ppb of Rb. Nymphs that fed as larvae on control mice contained approximately 9 ppb. There was no host-specific difference in Rb concentration among nymphs obtained from four RbCl-injected mice. Rb-marked nymphs did not show reduced survival as compared with unmarked nymphs when survival was monitored for 8 wk. Rb concentrations in unfed nymphs remained close to 600 ppb for 3 wk, then declined to approximately 380 ppb for 5 wk. Rb concentrations in experimental nymphs remained significantly higher than in control nymphs for 8 wk. Adults that molted from Rb-marked nymphs that had fed on control mice did not contain elevated levels of Rb. This marking method has considerable potential for aiding in population ecology studies of ticks under natural conditions because ticks can be marked via their hosts and the mark will be retained for the duration of the life stage without passing the mark to subsequent stages.