This study examined the effect of one interdependent group contingency intervention, the mystery motivator, as it affected three students identified as exhibiting problem behaviors, as well as the effects on nonidentified students in a ninth-grade high school biology class. An A/B/A/B single-case design was used to evaluate the effects of the intervention. Results indicated a decrease in problem behaviors for the three identified students, as well as a general decrease in problem behaviors for randomly selected, nonreferred students in the class. Implications, limitations, and further directions are discussed. (Contains 1 figure.)