Students benefit from positive sport experiences in physical education. If designed well, sport provides a social avenue for physical activity and strengthens student achievement in psychomotor (e.g., motor skill), cognitive (e.g., decision-making), and affective (e.g., personal and social responsibility) learning domains. Unfortunately, not all students receive quality sport instruction and many students fail to have positive sport experiences in physical education. The Tactical Games Model (TGM, Griffin, Mitchell, & Oslin, 1997) is an instructional model focused on improving student sport experiences. As a constructivist approach to teaching and learning sport, TGM reshapes sport lessons to allow students to experience small-sided games (Game 1), think critically about games playing (Q & A), practice aspects of playing (Situated Practice), and show improvement in games playing (Game 2). TGM literature includes practitioner reports about involvement (Berkowitz, 1996) and findings that show measures of game performance (e.g., skill execution, decision-making) during a TGM sport unit (Allison & Thorpe, 1997; Turner & Martinek, 1999). Limited data is available to explain how the constructivist nature of TGM influences motivation (Griffin & Patton, 2005; Rink, 2001). The purpose of this qualitative study was to examine motivation using situational interest theory (Chen, Darst, & Pangrazi, 1999; Mitchell, 1993) to interpret participant – learning situation (Game 1, Q & A, Practice, and Game 2) experiences during an eight-day TGM Ultimate Frisbee unit. The researcher acted as teacher-researcher and participants were 15 fifth graders (assigned to heterogeneous teams) and Mia, the regular physical education teacher and participant-observer. Data were collected using surveys, learning situation questionnaires, interviews, and systematic observations using the Game Performance Assessment Instrument (GPAI, Oslin, Mitchell, & Griffin, 1998). Data analysis incorporated open and axial coding (Strauss & Corbin, 1998), theoretical comparisons (Strauss & Corbin, 1998), and concept mapping (Rossman & Rallis, 2003). Findings show that participants': (a) participated in daily lessons regardless of gender, goal orientation, skill/effort level, and personal interest in Ultimate, (b) were excited to play games (Game 1, Game 2) because they wanted to move, liked Ultimate, and/or wanted to assess skills/playing, (c) required challenging conditions, positive competition, and/or individual/team success in order to have a positive participant-games playing experience, (d) entered Q & A and Practice expecting to learn something new, (e) stayed interested in Q & A if they received answers, learned facts/rules, and/or felt the discussion helped team, (f) remained involved in Practice if team worked well, task was fun, and/or they learned skill/strategy, and (g) perceived improvements in games playing (e.g., throwing). Mia concluded that participants: (a) were motivated to play, (b) were involved in the different learning situations, and (c) improved games playing during the unit. GPAI scores confirmed that participants improved at least one area of game performance (e.g., skill execution-passing) between Day 3 (week 1) and Day 7 (week 2).