The intensity and prevalence of parasitism by Ascogregarina barretti (Vavra) in Aedes triseriatus (Say) did not differ between tires and tree holes in field samples taken in September 1996. There was significant variation in the intensity of parasitism among containers that was not significantly correlated with the pH, conductivity, or temperature of the container water. In an experiment manipulating habitat drying, treatments had a significant effect on A. barretti infection of Ae. triseriatus, only during midsummer in one of two years. Containers maintained at maximal volume had the lowest prevalence of parasitism, and containers that dried out had the greatest prevalence. In this experiment, there was also a season-dependent difference in the intensity and prevalence of infection between tree holes and tires. The first larvae to reach the fourth instar in tires in the early summer had lower intensity and prevalence of infection than did larvae in tree holes during the same period. The seasonal difference in intensity of parasitism between tires and tree holes was not related to differences in pH, conductivity, and temperature.