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Geographical areas historically characterized by a lower risk of diurnal avian predation should in theory allow bats to be active in daytime too, especially to forage. Oceanic islands are ideal for studying temporal niche shifts in bats since they often feature depauperate avian assemblages with fewer birds of prey. We report on the second case of diurnal activity known for an insular insectivorous bat, Hipposideros ruber on the island of Sao Tome (Gulf of Guinea). We present observations of daylight flights made at several sites on the island in six months (from July to November and in January). We also carried out daytime (09h00–16h00) emergence counts at three roosts. In this time interval, bats were continuously active, although activity rates changed: bats tended to return to roosts in early afternoon and, at two sites, to decrease emergence rate under intense light probably to reduce the risk of hyperthermia. At one roost, we observed that heavy rain caused abrupt daytime returns of large numbers of bats. In daytime bats did forage and kept echolocating. Social interactions (chases) were also frequently observed. In daytime adult males were significantly more frequent than females, while the opposite was noticed at night, an intersexual segregation possibly leading to temporal niche partitioning.