We examine the interplay between unethical behaviour and competition with a lab experiment. Subjects play the role of firms in monopoly, weak competition (Bertrand–Edgeworth duopoly) or strong competition (Bertrand duopoly). Costs are determined either by a computer draw or a self-reported die roll, and pricing decisions are made with knowledge of one’s own costs and—in duopoly—the rival firm’s costs. Under self-reporting, lying is profitable and undetectable except statistically. We find that competition and lying are mutually reinforcing. We observe strong evidence that (behavioural) competition in both duopoly treatments is more intense when lying is possible: prices are significantly lower than when lying is impossible, even controlling for differences in costs. We also observe more lying under duopoly than monopoly—despite the greater monetary incentives to lie in the monopoly case—though these differences are not always significant.