The main concern of the present study was to probe the probable differences between Iranian bilingual / monolingual learners of English regarding their lexical and syntactic knowledge. It was an attempt to investigate whether bilingual I monolingual learners of English differ significantly in learning lexical and syntactic knowledge. To achieve this end, an ex post facto design was employed. A total of 150 female subjects at three pre-university centers of Tehran were randomly selected from among two groups of Turkish-Persian bilinguals and Persian monolinguals. One general English proficiency test, two questionnaires, and a grammaticality judgment test along with a correction task were administered to both groups. The results revealed significant differences between the two groups; that is, monolinguals outperformed bilinguals in both areas:
vocabulary and syntax. The findings have some implications for teacher training centers, methodologists, syllabus designers and curriculum developers.