Field and laboratory experiments were done to quantify the effects of high temperature and low soil moisture on the mortality of the immature stages of the carrot fly (Psila rosae F.). Field experiments related fly numbers to egg density and determined the period within each 8-10 week fly generation that resulted in most crop damage at harvest. Population monitoring: From 1995-1998, all stages of the carrot fly were monitored. No third fly generation was observed and no first-instar larvae were found after the end of September. Periods of high mortality were identified. [~gg mortality: Mortality was related linearly to the mean maximum soil temperature at 1 cm depth during egg development (m = 8.4t -167.3; m = % mortality, t = temperature). Under controlled conditions, eggs in dry soil died sooner at higher temperatures (>25°C) than eggs in moist soil. Fully developed eggs were more susceptible to adverse abiotic conditions than newly-laid eggs. Larval mortality: Up to 90% of first-instar larvae were killed by low soil moisture in the field. This mortality was related to the length of time larvae were exposed to dry soil conditions (m = 2.6 + 17.6d; m = % mortality, d = no. of days in dry soil). Relationship between numbers offlies and egg density: A linear relationship (e = 0.32 + 0.89[; e = log no. eggs, f = log no. flies) was identified between the numbers of carrot flies caught on sticky traps and the numbers of fly eggs recovered from carrot crops. Critical egg-laying period: Within a generation, the critical egg-laying period when most damage was caused was between 10-20 days. This coincided with the peak in fly numbers. Rearing method: A robust rearing method was developed for producing a regular supply of adults. Approximately 50% of eggs inoculated onto growing carrots gave rise to pupae.