Gregarious roosting behaviour and nocturnal activity is virtually unknown within the family Hesperiidae. Observations onCelaenorrhinus fritzgaertneri(Bailey) from Guanacaste Province, Costa Rica, are presented which demonstrate that, during the dry season, individuals roost gregariously in cave‐like situations during the day and are nocturnally active. Circadian behaviour is highly synchronized with regard to timing of departures and arrivals at the roosting site, and appears to be regulated by light levels. Individuals on the roost are in reproductive diapause and show marked roost‐site fidelity and longevity. The break‐up of roosts appears to be mediated by the onset of the first rains of the season. These observations are compared and discussed in relation to gregarious roosting in other Lepidoptera. Two possible life history strategies to explain roosting inC. fritzgaertneri, with appropriate experiments, are suggested for future st