This master's thesis investigated realizations of typologically marked structures (word final stops) in the interlanguages of 15 ESL learners across Arabic, Brazilian-Portuguese and Japanese first languages (L1s). In general, previous theories of markedness (see Eckman's MARKEDNESS DIFFERENTIAL HYPOTHESIS and STRUCTURAL CONFORMITY HYPOTHESIS) and transfer (such as Major's ONTOGENY MODELS) were upheld in that more marked structures proved more problematic than less marked areas. Where uniformity of modification strategies was found, OPTIMALITY THEORY was implemented to illustrate process of acquisition undertaken during interlanguage development. In an isolated speech task, participants who demonstrated acquisition of more marked structures (ie., voiced final stops) were also successful with their less marked counterparts (voiceless final stops), but not vice versa. In connected speech, more advanced participants' modifications of target structures (such as assimilation of voicing and place of articulation) were more similar to patterns exhibited by native speakers of the target language while less advanced participants' productions (ie., lack of intervocalic voicing) were more reflective of their L1. These findings support the hypothesis that interlanguages adhere to universal grammar and, thus, behave as natural languages. Finally, future directions such as potential research of L1/L2 perception issues and pedagogical implications of the study's results are explored.