Context: Gamification appears to improve enjoyment and quality of execution of software engineering activities, including software testing. Though commonly employed in industry, manual exploratory testing of web application GUIs was proven to be mundane and expensive. Gamification applied to that kind of testing activity has the potential to overcome its limitations, though no empirical research has explored this area yet.Goal: Collect preliminary insights on how gamification, when performed by novice testers, affects the effectiveness, efficiency, test case realism, and user experience in exploratory testing of web applications.Method: Common gamification features augment an existing exploratory testing tool: Final Score with Leaderboard, Injected Bugs, Progress Bar, and Exploration Highlights. The original tool and the gamified version are then compared in an experiment involving 144 participants. User experience is elicited using the Technology Acceptance Model (TAM) questionnaire instrument.Results: Statistical analysis identified several significant differences for metrics that represent the effectiveness and efficiency of tests showing an improvement in coverage when they were developed with gamification. Additionally, user experience is improved with gamification.Conclusions: Gamification of exploratory testing has a tangible effect on how testers create test cases for web applications. While the results are mixed, the effects are most beneficial and interesting and warrant more research in the future. Further research shall be aimed at confirming the presented results in the context of state-of-the-art testing tools and real-world development environments.