AbstractEphestia elutellalarvae in diapause were exposed at 25°C to methyl bromide at 12 mg litre−1for 3.5 or 7.5 h and then immediately exposed to a lower concentration. The minimum effective concentration (that at which Haber's rule, concentration × time =k, a constant for mortality, still applied) was about 3 mg litre−1in tests with no previous exposure toa high concentration, but it was about 2.5 mg litre−1for individuals surviving a 3.5 h exposure to 12 mg litre−1, and was about 1.6 mg litre−1for those surviving a 7.5 h exposure to 12 mg litre−1. These exposures to 12 mg litre−1, respectively, killed 2–20% and 50–75% of larvae exposed, and hence the smaller the proportion of survivors of exposure to a high concentration, the lower the minimum effective concentration needed against them. Thus the low concentration persisting at the end of a practical fumigation should contribute significantly to the success of the treatment and be much more effective than any similar low concentration present soon after the