Studies found EL learners overuse and as an additive connector at the sentence-initial position (Bolton, Hung, & Nelson, 2002), and they underuse and as a coordinator (Leung, 2005). Generally, the use of the and-coordinator has often been overlooked in corpus research and in English teaching because of its seemingly simplicity. To test previous findings about the and-coordinator and to examine the influence of English proficiency on the use of and in academic writing, three learner corpora--MICUSP-NNS (advanced level), MICUSP-NS (advanced level), and NUCLE-NNS (upper-intermediate) were compared, with regard to the use of (a)symmetric structures of the and-coordination. Each corpus contains 31 argumentative essays written by 31 university students.