OBJECTIVE: To examine protocol fidelity among teachers involved in a six-month cluster-randomized physical activity (PA) intervention. METHODS: In 2011, preschools in Springfield, MA were randomized to short bouts of structured PA (SBS-PA, n = 5) or unstructured playtime (UPA, n=5). SBS-PA provided structured PA in the classroom during the first 10 min of gross-motor playtime followed by 20 min of unstructured playtime. UPA consisted of 30 min of unstructured playtime. All teachers (SBS-PA and UPA) received a written study protocol and 1.5 h of training. SBS-PA also received videos to use to lead structured PA and 1.5 additional hours of training. Study fidelity and process evaluation were assessed twice weekly via semi-structured questionnaire. RESULTS: Only 56.6% of SBS-PA and 75.2% of UPA free playtimes lasted for 30 min; 86.3% of SBS-PA teachers implemented structured PA during the first 10 min of gross-motor playtime but only 67.2% delivered the intervention as instructed. Only 68.5% of SBS-PA teachers implemented the 20-minute unstructured playtime. SBS-PA teachers reported that time limitations was a major barrier in implementing the designed intervention. Pre-post changes in PA did not differ between groups. CONCLUSION: Limited fidelity to intervention protocol likely impacted study findings. Future studies should focus on strategies to improve adherence among intervention leaders.