Nationwide, K-12 students designated as English language learners (ELLs) must learn both language and content simultaneously, and ELLs score far below the national average in math achievement. Many educators have suggested that identifying or developing teachers with skills specific to ELLs' instructional needs may be critical to addressing this challenge. This study seeks to identify the characteristics and learning experiences of general education teachers who are differentially effective at promoting math achievement among ELLs compared to non-ELLs. Our analyses indicate that individual teachers can learn specific skills that make them more effective with ELL students. In particular, prior experience teaching ELLs predicts improvements in novice teachers' differential instructional effectiveness with ELLs. We also find that both in-service and pre-service training focused on ELL-specific instructional strategies are associated with higher differential gains for ELLs. Our findings lend support to the notion that general education teachers can develop valuable ELL-specific instructional skills.