Feeding activity is a crucial measure used in experiments with larvae of Pseudosarcophaga affinis auct. nec Fallén on chemically defined diets to determine whether food selection and preference occurs because of nutritional factors. To avoid temporal bias in recording it was necessary to know: (1) when during larval development feeding activity is greatest; and (2) whether a diel periodicity of feeding activity demands that observations be standardized according to time of day. Experiments done at 23 °C and 16-hour photoperiod with new-born larvae showed that: (1) during larval life feeding activity increases rapidly up to 96 hours of age and thereafter slowly decreases; and (2) on the two diets tested more feeding occurred during darkness but only on one diet was the difference (about 7.3%) statistically significant. Thus in P. affinis it is seldom necessary to control for time of day in experiments of the kind used in the work on food selection and preference.