In October 1961, 12.9 per cent of 62 _P. truei_ were infected with warbles. The average number of warbles per infected mouse was 1.37. Seventy-three per cent of the warbles were in the third instar stage; the rest were in the second instar stage. Warble infestation was higher in the first half of October (16 per cent of the mice infected) than in the second half of the month (5.5 per cent infected). These mice were collected from several localities on Chapin Mesa, in pinyon-juniper woodland. In Mesa Verde the greatest incidence of infestations is in late September and early October. This agrees with the finding of other investigators (Sealander, 1961:58). Sealander (1961) investigated hematological values in deer mice infected with botflies, and found that infected mice had significantly lower concentrations of hemoglobin than non-infected mice. Myiasis, associated with infection by _Cuterebra_, is likely to lead to a lowering of the physiological resistance of a segment of the population, and perhaps to a subsequent decline in the population (Sealander, 1961:60). Mice infected by warbles were less agile than non-infected mice. Other investigators also have reported awkwardness in locomotion in infected mice (Scott and Snead, 1942:95; Sealander, 1961:58). Test and Test (1943:507) noted that parasitized mice did not appear to be emaciated, and this was also true of parasitized mice at Mesa Verde. Healed wounds, where warbles had emerged, were apparent on a number of mice. The warbles, and wounds, usually were found on the flanks and backs of the mice. The large, third instar larvae weighed about one gram apiece; there is little doubt that such large larvae induce trauma in their hosts. The highest rate of infestation by botflies occurred in 1961, the year in which the population density of _P. maniculatus_ was near its peak. The population of this species was reduced considerably in 1962, and remained low through 1964. In 1965, the density of _P. maniculatus_ appeared to be increasing. Other investigators have reported that increased incidence of _Cuterebra_ infestation in deer mice coincides with lower population densities and with a downward trend in the population (Scott and Snead, 1942:95; Wilson, 1945). My data indicate that this may not be the situation in Mesa Verde. The intestines or stomachs of almost all individuals of _P. maniculatus_ contained parasites. Endoparasites were less abundant in individuals of _P. truei_. This heavier infestation of _P. maniculatus_ by tapeworms, roundworms, and spiny-headed worms probably reflects the larger proportion of insects eaten by _P. maniculatus_ than by _P. truei_.