Abstract Adults of a phosphine‐resistant strain of Sitophilus oryzae (L) were exposed to constant phosphine concentrations of 0.0035–0.9 mg litre −1 for periods of between 20 and 168 h at 25 °C, and the effects of time and concentration on mortality were quantified. Adults were also exposed to a series of treatments lasting 48, 72 or 168 h at 25 °C, during which the concentration of phosphine was varied. The aim of this study was to determine whether equations from experiments using constant concentrations could be used to predict the efficacy of changing phosphine concentrations against adults of S oryzae . A probit plane without interaction, in which the logarithms of time ( t ) and concentration ( C ) were variables, described the effects of concentration and time on mortality in experiments with constant concentrations. A derived equation of the form C n t = k gave excellent predictions of toxicity when applied to data from changing concentration experiments. The results suggest that for resistant S oryzae adults there is nothing inherently different between constant and changing concentration regimes, and that data collected from fixed concentrations can be used to develop equations for predicting mortality in fumigations in which phosphine concentration changes. This approach could simplify the prediction of efficacy of typical fumigations in which concentrations tend to rise and then fall over a period of days. Copyright © 2004 Society of Chemical Industry