The present study investigates the effect of different exposure densities on third grade Iranian junior high school (IJHS) students’ EFL development and retention. It also attempts to explore whether there is any differential performance due to type of post-exposure assessment tasks (recognition vs. written production). Performances of five groups of learners receiving equal amount of exposure to EFL knowledge with different densities over different time spans were compared on both recognition and written production tasks. The results of the repeated measures analysis of variance and paired sample tests reveal that there seems to be a threshold exposure density for development of structure production, as well as for the retention of total EFL and vocabulary production. Analyses of variance for the five groups were performed on the raw scores obtained for a pre-test and two post-tests. The results further reveal significant differences for acquisition and retention of the target EFL knowledge among different groups. The results also suggest different optimal exposure densities for the target EFL knowledge development and retention.
