The effects of temperature, relative humidity, and concentration and amount of trypticase soy broth (TSB) on regurgitation byMusca autumnalisDeGeer were determined. The highest amounts and concentrations of TSB resulted in the most frequent regurgitation. Regurgitation occurred most often at the lowest relative humidities (RH) examined, and regurgitation was most pronounced at 29°C. Significant interactions between RH and concentration of TSB, RH and amount of TSB, concentration and amount of TSB, temperature and concentration of TSB, and temperature and amount of TSB were found.