This study identified important questionable behaviors conducted by tour group leaders, tour guides, travel agency reception personnel, and other tourists in group package tours and investigated how tourists perceive those questionable behaviors. Data for this study were obtained from tourists who had taken part in a Taiwanese group package tour in the recent year. Results of the study show that, leading to damage on others’ properties, violating principles on information giving and handling, inappropriate voluntary behavior, and violating pre-set itinerary were mostly perceived by tourists as inappropriate. Significant correlations between certain questionable behaviors and tourists’ general attitudinal statements were also verified. This study is beneficial in helping both academia and industry personnel better understand the behaviors that affect tourists’ perceptions.