Field populations of Lucilia cuprina (Wied.) in New South Wales were sampled for periods of 3 h on numerous occasions between 1975 and 1982 using West Australian blowfly traps. Ambient temperature, wind speed, relative humidity and solar radiation explained 77·4% of the within-day deviance of the catches. Temperature alone accounted for 74·9% of this deviance, indicating that the other variables, although significant, did not greatly affect trap catches. As air temperatures increased, log catch rates increased linearly up to 26°C and then remained constant up to 35°C. Wind speeds above 25 m/s caused a linear decline in log catch rates. Log catch rates increased linearly as solar radiation increased and decreased linearly as relative humidity increased. Changes in catch rates with time of day were explained almost entirely by the four weather variables, i.e. there was no evidence that intrinsic behavioural changes with time of day affected catch rates. The combined effects of the four weather variables accounted for 47·1 % of the between-day variation in trap catches. Trap catches that have been standardised, i.e. adjusted to a ‘ standard’ set of weather conditions, provide relative measures of population size which differ from absolute measures by a constant (unknown) scaling factor.