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. (author abstract)