Immature blow flies (Diptera: Calliphoridae) collected from decomposing human remains are often used to determine the minimum postmortem interval (PMImin). Phormia regina (Meigen) is a common blow fly of cosmopolitan distribution that is often associated in such cases. P. regina development at two different cyclic temperatures was examined in this study. A field validation study was conducted to determine the accuracy of applying these data to determine the PMImin. Minimal total development time was 32.52 d at cyclic 14.0 ±2.0°C and 16.60 d at cyclic 20.5 ±3.1°C. The minimal larval development was significantly different (P < 0.05) across temperatures. Larval development needed 15.5 d at 14.0°C and 7.5 d at 20.5°C. For the validation study, instar, mean, and maximum of length and weight data of the larvae collected in the field were analyzed with data generated from the 20.5°C treatment, as it more closely reflected the field conditions experienced. Accuracy in estimating PMImin, was highly variable depending on the unit of measurement used and instar of P. regina collected from the field. Using the oldest instar to estimate a PMImin resulted in ranges that always encompassed the true time of colonization. Accuracy in hours when using measurements units as mean length or weight, and maximal length or weight, varied among the larval instars. In the first instar the greatest overestimation was made with maximal weight while the greatest underestimation was made with mean weight. The most accurate estimate produced with first instars was based on maximal length. In the second instar, there was no overestimation and the greatest underestimation was made with mean weight and the most accurate estimate produced was with maximal length. In the third instar, the greatest overestimation was made with maximal length, and the greatest underestimation was made with mean weight. The estimated time of colonization based on maximal weight was most accurate for third instars.