This paper examines L2 acquisition of stress in Khalkha Mongolian, which is one of the few Default-to-Opposite Edge stress systems of the world, and as such, demonstrates "conflicting directionality" regarding stress assignment. Given the additional fact that the language also exhibits Nonfinality effects, and that, unlike English, codas are not moraic, despite being a weight-sensitive language, its acquisition presents unique challenges for English-speaking learners of the language. Many of these challenges lead these learners to make UG-unconstrained (but cognitively reasonable) assumptions about how the phonology of Mongolian works, and the learning scenario here, thus, provides unique opportunities to investigate whether L2 acquisition is guided by the options made available by UG. The findings of a semi-controlled production experiment which was designed to test these issues indicate that the hypothesis space for interlanguage phonologies is determined by UG. Although learners do not necessarily converge on the prosodic representations employed by native speakers, they demonstrate a stage-like behavior where each step exhibits the parameter settings employed by a natural language, one that is neither like the L2 nor the L1. Further, despite the input leading them to do so, learners do not entertain UG-unconstrained prosodic representations, another fact that is taken here as evidence for access to UG in L2 phonology.