SummaryEggs, larvae, pupae and adults of the large narcissus fly (Merodon equestris) were reared at a series of constant temperatures between 9–24°C. Egg development required from 37 days at 9°C to 7 days at 21.5°C. The low‐temperature threshold for development was 6.7°C. Larvae reared at 1424°C were fully‐grown after 18 weeks, but it took much longer for such insects to pupate, and adult flies emerged only after about 45 weeks of development. Large narcissus flies enter diapause during the larval stage and overwinter as fully‐fed larvae, forming pupae in the following spring. Post‐winter pupation and pupal development took from 169 days at 10°C to 36 days at 21.5°C. Of this, pupal development required from 91 days at 10°C to 19 days at 21.5°C. The low‐temperature threshold for post‐winter pupation and pupal development was 7.1°C, and for pupal development alone, 7.2°C.Females maintained at or below 19°C laid few eggs, whereas some females kept at or above 21.5°C laid more than 100 eggs (mean 69 ± 36). Approximately 50% of females maintained at or above 21.5°C laid less than 10 eggs during their lifetime. The mean egg‐laying time was 6 to 9 days. Although temperatures at or below 19°C inhibited mating, once a female had mated, such temperatu