AbstractPulses of various durations at temperatures between 29 and 38°C were applied to developing larvae ofDrosophila melanogastercarrying the temperature‐sensitive cell‐lethal mutation1 (1)ts726. The results show that it is not possible to reduce the time required for the induction of abnormalities in the mutant by treating larvae with heat pulses at temperatures higher than 29°C. Instead, treatment with high temperature leads to fewer abnormalities than 29°C treatments. Furthermore with high temperature treatments, the mutation has less effect on viability than is seen at 29°C. It is suggested that1 (1)ts726leads to abnormalities and death by a temperature‐induced imbalance between different physiological or development events, rather than by interfering with the ability of the cell or the organism to withstand high temperature i