New studies show that the instructional effectiveness of preservice candidates and their cooperating teachers are positively related. However, we neither know if these relationships are causal nor, assuming they are, if it is possible to significantly increase the instructional effectiveness of the cooperating teacher pool. In this study, we randomly assign districts to receive recommendation lists (generated using administrative data) for the recruitment of more promising cooperating teachers. Districts receiving lists recruited significantly more effective/experienced cooperating teachers, while candidates placed in these districts felt significantly better prepared to teach. As a result, this study offers an innovative, low-cost strategy for recruiting effective/experienced cooperating teachers and presents the first causal estimates that more effective/experienced cooperating teachers improve candidates' preparedness to teach.