A physiological day equation proposed by Waddill et al. (1976) of the form PD = 1/24 {[.55(TMAX−TMIN)+TMIN]−THT+[.15(TMAX−TMIN)+TMIN]−THT} where TMAX = Maximum Temperature, TMIN = Minimum Temperature and THT = Developmental Threshold, predicted development of the larvae of the Mexican bean beetle, Epilachna varivestis Mulsant, on soybeans and snap beans with only ca. a 2% error. This sine curve based equation incorporates geographical location coefficients, and predictions were accurate enough for acceptable simulation limits, especially with fluctuating temperatures such as those that occur in the field. Predictions by the equation were improved when temperatures did not approach the physiological limits of beetle development. The total number of days required for larvae to mature on snap beans ( Phaseolus ) and soybeans was 16.5 and 23.5, respectively. Extension of Mexican bean beetle developmental time when soybeans were utilized as food may have serious implications when compiling biological data for computer simulations.