English language learners (ELLs) are the fastest growing segment of the U.S. school population, which means schools are hiring significant numbers of new ESL (English as a Second Language) teachers. This burgeoning pool of ESL teachers must be supervised by administrative staff, and the stakes are considerably higher now that teacher-evaluation policies frequently require teachers to make the grade or lose their jobs. But most administrators' educational experiences are remote from ESL instruction; few administrators are former ESL teachers, and supervisory training routinely fails to encompass ESL pedagogy. Hence, it remains unclear whether the administrators who supervise ESL teachers feel competent to do so. It seems plausible that the increasing ESL population is causing a supervision problem in modern schools: more and more ESL teachers whom administrators feel unprepared to supervise. To test this theory, a study was conducted focused on administrators' self-efficacy beliefs in supervision of ESL teachers. We designed a new survey instrument and evaluated its psychometric characteristics with a sample of 75 administrators, with linear regression performed to explore factors that predict administrators' self-efficacy beliefs in ESL teacher supervision. Results indicate that the more ESL teachers an administrator supervises, the lower the self-efficacy the administrator reports in supervising these teachers. So the increasing quantity of ESL teachers is in fact producing a growing problem in schools, indicating an urgent need for more extensive and higher-quality training for administrators in the objectives and methods of ESL instruction.