Abstract Time as a response factor was studied in adults of Tribolium castaneum (Herbst) over a range of fixed concentrations of phosphine from 0.0011 to 9.9 mg/l. At concentrations where response time could be separated satisfactorily from the time component of dosage, and for dosages producing up to 100% mortality, the median response time (MRT) was inversely correlated with dosage and directly correlated with concentration. Thus the speed of action of phosphine over this dosage range increased with increasing period of exposure for a given concentration, but decreased with increasing concentration. Mean times to end-point mortality, derived as post-treatment holding periods for each fixed concentration, were directly correlated with log-concentration. The usage of end-point mortality in toxicity measurements is discussed.