This study investigated the effects of naturalistic, teacher-mediated intervention strategies on increasing levels of active engagement in three children with developmental delays. Three teachers participated and a multiple-probe design was used with three subjects. All interventions occurred in the context of ongoing child-selected play activities in preschool classrooms. The rate of engagement were observed in baseline and experimental conditions occurred during 10-minute sessions in which all children were free to select different activities. Results showed that the intervention strategies were effective in increasing the level of active engagement in identified areas of need for all three children. These findings suggest that an impor-tant ingredient in the successful application of systematic instructional procedures in classroom settings may be related to how proficient teachers are at organizing and constructing an instructional trials. The effectiveness of the naturalistic intervention and the applications of the naturalistic intervention to the other ongoing routines in preschool are discussed.