Aims The basis of this study lies in hygiene of swimming pools, where it is argued that taking a pre-swim shower would lead to a decrease in release of pollutants in the swimming pool, which in the end would lead to the decrease of health issues as red eyes and asthma. However, previous research has shown that not even half of the swimming pool visitors takes a pre-swim shower. Therefore, interventions are needed to increase pre-swim shower behaviour. Whereas previous research focused on ‘conscious’ behavioural interventions, this research focuses on ‘unconscious’ behavioural interventions, by using watching eyes. Watching eyes create an unconscious feeling of being watched, which lead people to behave in a prosocial way (for example: people litter less). It is argued that when swimming pool visitors are ‘confronted’ with watching eyes, they are more likely to take a pre-swim shower. So, this study aimed to increase pre-swim shower behaviour in swimming pools by using watching eyes. Methods Three interventions were designed, either containing watching eyes, a symbol, or a combination of both (see figure 1). This led to three conditions: eyes-only (no symbol), eyes + symbol, and symbol-only(no eyes). A fourth condition was added in which nothing was displayed (no eyes/no symbol): the control condition. Three methods were used to gather data for this study: observations, questionnaires and water quality measurements. Observations were used to monitor actual pre-swim shower behaviour, and the influence of factors as gender, age, carrying belongings, other people showering, et cetera. These factors were found to influence pre-swim shower behaviour in previous research. Questionnaires were used to monitor hygiene perceptions, importance of hygiene, and hygienic behaviour, and about the awareness and understanding of the interventions. It was expected that data about the factors mentioned before could give more insights about how swimming pool visitors feel about hygiene in swimming pools, and about the influence of the interventions. Finally, water quality measurements were used to monitor the release of pollutants in the swimming pool. @en