Pterins are nitrogenous by-products of metabolism deposited in the eye of adult muscoid flies. Because pterin accumulation is positively correlated with time and temperature, measuring pterin levels in the adult head is a useful tool for determination of fly age if thermal accumulations above a lower threshold are known. F-1 generation adults of wild caught Musca domestica were held at 12, 15, 20, 25 and 30°C for 1-21 days post emergence. Head width, sex, temperature, age in days and amount of pterin measured were used to develop a regression model for each sex. This model, combined with temperature data, is used to determine the age of wild caught house flies collected from dairies in California, Minnesota and Georgia. This information will be discussed in the context of fly control in confined livestock facilities.