Abstract: The problem. The first session in brief strategic therapy is the most critical phase of treatment. More than a mere 'assessment phase,' the first session in brief-strategic therapy is a true intervention that sets the stage for all subsequent therapeutic maneuvers. Therapist proficiency in this session forms the cornerstone upon which successful treatment rests. The purpose of this study was to develop a supervisory observation tool to measure proficiency in specific skill sets necessary for fidelity in brief strategic therapy first sessions. A new fidelity measure, the Brief Strategic Therapy Scale-1 (BSTS-1), was proposed as a more formal method of analyzing performance and competency for the first session. Method. A multi-step procedure was implemented to develop this scale. First, the 6 underlying dimensions of the scale were defined and a focus group of expert judges rated how well the definitions fit the dimensions and how the 6 dimensions might be incorporated into phases of the first session. Second, the focus group sorted the skill definitions into the known phases of first-session treatment and constructed statements that fit descriptions of types of behaviors. Third, the focus group of expert judges sorted these statements into a ranking from very deficient to very proficient for each phase of the first session. Fourth, the preliminary BSTS-1 was devised and disseminated to a wider sample of expert collaborators who offered general feedback on its utility. Fifth, the final BSTS-1 was developed. Results. The results of this study produced a measurement to assess the competency of brief strategic therapists in specific skill sets and their fidelity to the model. The BSTS-1 was divided into a series of 5 subscales, each focusing on a specific task associated with a phase of the first session: questioning capacities, paraphrasing to reframe, evoking sensations, summarizing to redefine, and prescriptions. Future research should focus on empirical investigations of the BSTS-1, its psychometric properties, and validation of the scale.