Many studies of canopy arthropods in rainforests rely on methods that do not permit comparison of day and night activities of the target fauna: accordingly, data on arthropod diet activity are scarce. Nevertheless, such information as is available suggests significant dissimilarities during the course of the day and a greater activity of insect herbivores during the day than at night. We studied faunal exchanges, particularly of insect herbivores, between day and night, and between the upper canopy and understorey of a lowland rainforest in Gabon. In total 14 161 arthropods were collected by beating, flight interception and sticky traps from six canopy sites during mid January to mid-March 1999. Diel activity explained about 6 - 9% of the total variance in arthropod distribution, depending on the sampling method. In general, arthropod activity was higher during the day than at night. In particular, Thysanoptera, Psylloidea, Membracidae, Curculionidae, Scelionidae and Apidae were notably more active during the day in the upper canopy. Similarity values between day and night faunas from particular strata were often higher than between faunas of the understorey and the upper canopy at a particular time of day. This suggests that there are fairly distinct suites of herbivores foraging in the understorey and in the upper canopy. Faunal turnover between day and night was higher in the upper canopy than in the understorey, suggesting that changes in the micro-climatic conditions between day and night in the upper canopy may be more severe than in the understorey, and that only a well-adapted fauna may be able to cope with these changes. In addition, certain groups, such as the Chrysomelidae, may migrate during the night from the understorey to the upper canopy, although there was no evidence of such mass insect movement overall. Our data suggest that insect herbivores of the upper canopy may be resident and well adapted to environmental conditions there.