The current study attempts to put to the test the nature of interdependence between L2 and L1 in a bilingual mind. It aims to find out if reading strategy instruction (RSI) in L2, a) affects reading comprehension ability and attitude toward reading both in L2 and L1, and b) if so, in which language (source (L2) or target (L1)) each of these two dependent variables are more affected. To meet the purposes of the study, a quasi-experimental design was employed. Measures of reading comprehension and attitude toward reading in L2 and L1 were distributed as pretests and posttests to 48 Iranian EFL learners. The experimental group received RSI in L2, but the control group received traditional reading instruction. Reading comprehension and attitude toward reading improved significantly both in L2 and L1 for the experimental group, but not for the control group; however, for the experimental group the magnitude of improvement in reading comprehension was more in L1 than in L2, and the magnitude of improvement in attitude toward reading was more in L2 than in L1. Therefore, although languages are interdependent in one mind, cross-linguistic influences are not the same in magnitude in terms of the languages and domains involved.