Aim: To describe the development of an intervention-specific fidelity measure and its utilization and to determine whether the newly developed Sitting Together and Reaching to Play (START-Play) intervention was implemented as intended. Also, to quantify differences between START-Play and usual early intervention (uEI) services. Method: A fidelity measure for the START-Play intervention was developed for children with neuromotor disorders by: (1) identifying key intervention components; (2) establishing a measurement coding system; and (3) testing the reliability of instrument scores. After establishing acceptable interrater reliability, 103 intervention videos from the START-Play randomized controlled trial were coded and compared between the START-Play and uEI groups to measure five dimensions of START-Play fidelity, including adherence, dosage, quality of intervention, participant responsiveness, and program differentiation. Results: Fifteen fidelity variables out of 17 had good to excellent interrater reliability evidence with intraclass correlation coefficients (ICCs) ranging from 0.77 to 0.95. The START-Play therapists met the criteria for acceptable fidelity of the intervention (rates of START-Play key component use [greater than or equal to]0.8; quality ratings [greater than or equal to]3 [on a scale of 1-4]). The START-Play and uEI groups differed significantly in rates of START-Play key component use and quality ratings. Interpretation: The START-Play fidelity measure successfully quantified key components of the START-Play intervention, serving to differentiate START-Play from uEI.