This study explored the impact of a supplemental reading intervention delivered in English in Grade 1 on the performance of at-risk children educated in French Immersion schools. The intervention contrasted "Direct Mapping and Set-for-Variability" with a "Common and Best Practices" taught control condition in a matched quasi-experimental design. To test claims of cross-linguistic transfer, measures of English and French word reading were administered before and after intervention. Hierarchical linear modeling analyses confirmed that children in the intervention condition showed improved performance at posttest on measures of English and French regular, exception, and pseudoword reading compared to the control condition. Intervention-specific cross language effects on French word and pseudoword reading shown here provide partial support for causal models of transfer in bilingual reading development.