Preventive interventions for reducing occupational risks and health problems among farmers require the identification of factors contributing to unsafe behaviour, but research on this topic is rather limited. A theoretical model for studying factors affecting farmers' use of occupational safety and health (OSH) practices in Iran was developed. The model was empirically tested using data collected from a survey of 301 tomato farmers of Zanjan Province of Iran. The examined OSH practices encompassed a wide range of behaviours, grouped in four categories, i.e., use of pesticides, use of machinery, use of personal protective equipment (PPE), and applying ergonomic principles (i.e., fitting the task to the individual, designing the workplace based on human factors, taking into account the interaction between the workplace and the workers, exercising during work or rest). Almost half of the farmers (49.5%) showed unsafe behaviour in the use of PPE. Moreover, significant proportions of the farmers showed potentially unsafe behaviour in the use of pesticides (42.2%), in applying ergonomic principles (40.2%) and in the use of machinery (35.9%). Attitude towards OSH practices, knowledge on OSH practices, and self-efficacy in safety had a direct positive effect on farmers' use of OSH practices, explaining 73% of the variance in farmers' safety behaviour. Overall, findings contribute to a better understanding of the use of OSH practices among farmers, providing empirical evidence in the cognitive processing of farmers' with respect to safety behaviour in farming and offering practical information that can be incorporated into OSH intervention programs in Iran and other developing countries. (C) 2017 Elsevier B.V. All rights reserved.