Territoriality in the flesh fly Sarcophaga crassipalpis (Diptera: Sarcophagidae) is studied in the laboratory. In rectangular enclosures, male flies exhibit a lower tolerance (occupation of the same physical space) of same-sex conspecifics than do females. In circular arenas, male flies show significantly higher levels of spatial separation among themselves (as determined from nearest neighbour analyses) than do females: males show a slight tendency towards a uniform distribution, whereas females exhibit a slight tendency towards clustering. The male spatial behaviour occurs during the photophase but not the scotophase, suggesting that visual cues are required for maintenance of inter-individual spacing. No significant differences in male spacing behaviour occur between subjective day and subjective night in either constant dark or constant light conditions, suggesting that spatial patterning is not driven by a circadian rhythm.