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Most terrestrial animals live in natural atmospheric conditions, but some are also adapted to low oxygen (hypoxic) or high‐carbon dioxide (hypercapnic) conditions, such as in social insect nests, soil, caves, wood, and decaying material. Although it is possible that individuals adapt their behaviour to the environmental condition of their habitats, mating behaviour under ecologically possible ranges of CO2 has not been well studied. We compared walking activity, duration of mating behaviour, and sperm transfer ability in the West Indian sweet potato weevil, Euscepes postfasciatus Fairmaire (Coleoptera: Curculionidae), in high CO2 (10 000 p.p.m.) vs. normal atmospheric conditions (laboratory air, ca. 800 p.p.m.). We found that high CO2 enhanced walking activity and mounting frequency. Under such circumstances, we predicted that enhanced activities under a high‐CO2 environment would increase the risk of sperm competition, which induces an extended copulation period and an increase in sperm transfer. However, weevils shortened the mating period and did not alter their sperm transfer ability under high CO2. These findings are, as far as we are aware, the first report of the effect of ecologically relevant high CO2 on insect mating behaviour under mass‐rearing conditions. The effect of ambient conditions on mating behaviour and sperm transfer is discussed in relation to the intensity of female refusal behaviour directed against males. [ABSTRACT FROM AUTHOR]