Abstract Behavior analysts have spent relatively little time in designing interventions to enhance motor development in typically developing infants and children. This study examines the effect of a motor training package consisting of opportunity to respond and practice (standing the infant and letting her hold the fingers of the experimenter), conditioned leg kicks (using Fischer Price's Kick and Play) and buttocks and trunk lifts (stimulated by contingent imitation of the infants responses) on minimally assisted standing behavior in a three month old infant. Using an ABAB reversal design, we were able to dramatically increase the amount of time the infant stood and decrease the infant's rate of wobbling while standing. Future directions for this research are discussed. Key words: Opportunity to respond, practice, standing, infant, cusp skills, and motor development ********** Thelen and Fisher (1982) reignited interest in environmental variables that affect infants motor development. Behavioral theories of development tend to hold that development is the product of person-environment interactions (Novak & Pelaez, 2004) and that learning plays a central role in development (Gewirtz & Pelaez-Nogueras, 1994). While behavior analysts and behavioral educators have completed research on many areas of development including adaptive physical education with school aged children (Magill, 1993; Schmidt, 1991), infant motor development appears to have lagged behind. One concept that holds considerable promise for infant motor development is opportunity to respond (Greenwood, Carta, Hart, Kamps, Terry, Arreaga-Mayer, Atwater, Walker, Risley, & Delquadri, 1992). Opportunity to respond has potential in facilitating children's language development (Hart & Risley, 1995) and has significant implications for cognitive development (Greenwood, Delquadri, & Hall, 1984; Greenwood, Hart, Walker, & Risley, 1994). However, a literature search on the term, using psycho-info, produced no results for the area of motor development. We became interested in the subject matter of opportunity to respond, as well as sensory reinforcement for motor behavior, because it was: (1) simple enough to track; (2) susceptible to environmental influences; and (3) a fun series of activities for parents to do with typical infants. The current study represents an on going extension of our work on motor development. Methods Participant Participant: is a three-month-old female. She is typically developing. She is the same subject as previously described in Cautilli and Dziewolska (2005). At birth subject was 9lbs 15 ounces and 22 inches placing her in the 99%ile for weight and 98%ile for height. At the time of the study she was approximately 18lbs and 26 inches long. Procedure Intervention Package The package consisted of a daily routine. This routine included: --Two to three periods of approximately 20 minutes in the Fischer Price Kick and Play[c]. The Kick and Play uses visual and auditory reinforcement in the form of flashing lights and common children's tunes for kicking responses. The tunes are played on an intermittent schedule with brief segments of sound played for kicks that do not achieve full reinforcement of a song. --10-15 standing episodes. These episodes involve the infant grabbing on to the two index fingers of the experimenter. The infant is then lifted to the standing position. The fingers represent an assist to give the infant an opportunity to respond. While in the standing position, the experimenter would look very excited by raising eyes, making exaggerated facial expressions, and talking to the infant--praising her for standing. --Two 15-minute episodes of contingent imitation for trunk and buttocks lifting. This procedure was identical to the procedure used in Cautilli & Dziewolska (2005). …