This experimental study investigates Persian learners’ clustering acquisition of overt obligatory subjects (PRO) and PRO in infinitival clauses in English as L2. Research has indicated that there are certain correlations between superficially unrelated linguistic phenomena in L1 acquisition that are interpreted as the clustering effects of one on the other. For instance, in German L1 acquisition, there is evidence for a clustering acquisition of subject-verb agreement and the decrease of (incorrect) null subjects (Clahsen and Hong, 1995). So, it might be argued that the same correlations happen in L2 acquisition. As a result, the present research will report on a grammaticality judgment test (GJT) investigating the clustering appearance of obligatory overt subjects and infinitival clauses in 60 Persian learners of English divided in three PROficiency levels, each level consisting of ten early starters and ten late starters. Our findings show that the two phenomena accompany each other in the Persian learners indicating that overt obligatory subjects and infinitival clauses in L2 are acquired through the PROcess of clustering effects, rather than separately. Moreover, we observed no difference between the performance of early and late starters with respect to clustering acquisition of the above linguistic variables.