Going beyond the rational choice approach used in conventional economics of crime, the question arises whether psychological personality traits analysis can contribute to a better understanding of non-compliance and, eventually, to the prevention of illicit behaviours. This study investigated how personality traits are related to compliance with environmental regulation in agriculture. The object of study was a water-protection rule that required farmers using fertilising to keep it a minimum distance away from nearby water bodies. Self-interested infringements can cause serious environmental damage to waters (negative externalities) through nitrogen runoff. In a survey among German farmers, we employed a 10-item version of the Big Five Inventory to measure the traits that are used as predictor variables in a regression analysis. The outcome variable was the farmers’ compliance behaviour in a business management game where rule-breaking was more profitable than rule-abidance. Some noteworthy findings were observed in the surveyed sample. (i) Neuroticism was positively related to ‘overall compliance’, measured as a binary yes/no variable; that is, more anxious farmers were less prone to rule-breaking. Surprisingly, however, a positive relationship between neuroticism and compliance was not found when looking separately at the deviant subgroup; here, greater neuroticism was associated with more severe rule violations, in terms of illicitly fertilised acreage. (ii) In the deviant subgroup, as might have been expected, higher levels of conscientiousness were associated with less severe rule-violations. Contrary to expectations, again, higher levels of agreeableness were linked to more severe non-compliance. A substantial positive relationship was found between extraversion and the severity of non-compliance, in accordance with ex-ante expectation. For openness to experience, no noteworthy results were obtained. The results indicate that agents with heterogeneous personality traits might react differently to identical economic and institutional environments. Moreover, it is suggested that, other than traits, there is another quality in agents (e.g. social control) that may have a decisive influence on their belonging to the compliant or non-compliant subpopulation. Farmers’ responses to changes brought forward by regulators who aim to prevent rule-breaking might therefore differ as well.