Emotional state may influence cognitive processes such as attention and decision-making. A cognitive judgement bias is the propensity to anticipate either positive or negative consequences in response to ambiguous information. Recent work, mainly on vertebrates, showed that the response to ambiguous stimuli might change depending on an individual's affective state, which is influenced by e.g. the social and physical environment. However, the response to ambiguous stimuli could also be affected by the individual's behavioural type (personality), a question that has been under-investigated. We studied the link between individual differences in exploratory activity and the response to an ambiguous stimulus in the ant Camponotus aethiops. Exploratory behaviour, quantified with an open-field test, was variable among individuals but consistent over time within individuals. Individual ants learned to associate a spatial position to a reinforcement and another spatial position to a punishment. Once the ants had acquired this discrimination, cognitive judgement bias was tested with the stimulus in an intermediate position. Fast explorers in the open-field took significantly more time to approach the ambiguous stimulus compared to slow explorers, suggesting a negative judgement bias for fast explorers and a positive bias for slow explorers. This previously unknown link between individual difference in exploratory activity and cognitive bias in a social insect may help understanding the evolution and organization of social life.