Cross-language effects on reading skills are of particular interest in the context of foreign language immersion programs. Although there is an extensive literature on cross-language effects on reading in general, research focusing on immersion students and including different dimensions of reading acquisition such as reading fluency and reading comprehension is scarce. This study therefore investigated cross-language transfer between first-language (L1) and second-language (L2) reading fluency and reading comprehension in a group of 220 German elementary school students who were enrolled in English partial immersion programs. Students were tested in grades 3 and 4. Structural equation modeling was used to examine cross-language transfer in a cross-lagged panel design. Results showed moderate cross-language paths when controlling for autoregressive effects. These findings are in line with previous results showing reciprocal transfer effects between L1 and L2 reading comprehension and reading fluency. In addition, the overall dominance of paths from L2 to L1 over paths from L1 to L2 suggests immersion-specific relations that may be attributable to the plentiful opportunities for academic reading in the L2 at school. Hence, skills necessary for successful reading can evidently be acquired in an L2 context and transferred to the L1. These findings underline the importance of cross-language transfer between reading skills in immersion programs. (Contains 1 table and 1 figure.)