In recent years the McDaniel mite has become a serious pest of fruit trees in Utah. This mite has been collected on most varieties of fruit trees in all the major fruit growing areas of Utah. In addition to these fruit tree hosts, it has been found on many weeds occurring in the cover crops of the orchards. These mites overwintered as orange-colored adult females under the bark or in the soil at the base of the fruit trees. Upon emerging in the spring they went either to the young leaves at the base of the fruit trees or began feeding on weed hosts. The life stages of this mite include the egg, larva, protonymph, deutonymph, and adult. During hot weather the mites required only about 8 days to develop from eggs to adults. In the field, temperature had a marked effect on the rate at which the McDaniel mite reproduced. If the weather became cold after the mites had become established the populations would remain almost constant until the temperature rose again. Laboratory experiments showed that there was a definite correlation between the number of eggs laid daily and the minimum daily temperature. Temperature had a marked effect on the duration of the preovipositional and ovipositional periods, the number of eggs laid daily, and the total number of eggs laid per female. The optimum temperature for the production of eggs appeared to be between 29° and 32°C. The host plants used influenced the development of the mites, the duration of the preovipositional and ovipositional periods, the number of eggs laid, and the proportion of mites failing to reach maturity.