Behavioural preferences of Drosophila larvae are often measured in a simple 'plate test', in which groups of larvae are allowed to migrate on an agar surface in response to variation in a stimulus. We investigated the behaviour of individual larvae in an olfactory test, alone and in groups. Using mixed groups of responsive and nonresponsive larvae, we showed that, although larvae behaved as individuals, the strength of their responses was affected by the presence of other larvae. This effect was apparently due to larvae bumping into each other, which reduced the distance they travel in the test. Larval olfactory responses were not affected by starvation: there were no significant differences in the responses of starved and not-starved larvae up to a duration of 5 h without food. However, food deprivation did significantly affect the movement of larvae, with more starved larvae leaving the start zone. We found significant differences in the olfactory responses of maggots of different ages, with responses increasing in strength with increased age but declining at the oldest age tested (late third instar). Finally, we found evidence that suggests that larvae may show a circadian modulation of the strength of their olfactory response, with a significant increase over the light:dark cycle. This response was maintained when wild-type larvae were shifted to continuous darkness but not in arrhythmic period0 mutant larvae. We hypothesize that this apparent effect is due to a central modulation of locomotor behaviour rather than to a specific effect on olfaction. © 2007 The Association for the Study of Animal Behaviour.