Modes of ectoparasite reinfestations were studied on ectoparasite-free deer mice (Peromyscus maniculatus) returned to their natural habitat on the Tule Lake National Wildlife Refuge, Siskiyou County, California, during the summer of 1977. The age of the host made no significant difference in the mode of reinfestation of lice, fleas, or mites. Flea reinfestation rates were related to the sex of the host, requiring 4 and 2 days, respectively, to reach control levels on male and female hosts. Mite populations reached the control level within 1 day, regardless of the sex of the host. No statistically significant louse reinfestations were noted within 8 days after the hosts were released. The percent of the host population reinfested with each ectoparsite followed the same patterns of reinfestation as the numbers of each parasite per host. It is suggested that the mode of ectoparasite reinfestations is a function of the behavior of the host relative to the lifestyles of the ectoparasite species.