Motivational interviewing, which involves the use of person-centered, directive counseling techniques, shows promise for changing adolescent behaviors. The purpose of this article was to describe the methodology and findings related to the treatment fidelity of three face-to-face motivational interviewing sessions involving middle school girls and a school nurse to help the girls increase their moderate-to-vigorous physical activity. The following four areas related to treatment fidelity were addressed: (a) study design, (b) training of interventionists, (c) intervention delivery, and (d) intervention receipt. Findings showed that 34 of 37 (91.9%) girls completed all three sessions. An initial motivational interviewing training workshop followed by evaluation of audiotaped sessions with constructive feedback can result in successful and consistent delivery by a school nurse. (Contains 4 tables.)