Implicit and explicit knowledge of the second language (L2) are two central constructs in the field of second language acquisition (SLA). In recent years, there has been a renewed interest in obtaining valid and reliable measures of L2 learners' implicit and explicit knowledge (e.g., Bowles, 2011; R. Ellis, 2005). The purpose of the present study was to examine the nature of the knowledge representations developed by two groups of learners of Spanish as a L2 at different levels of proficiency. The results show that the two groups differed with respect to their implicit and explicit knowledge of Spanish and also regarding the relationship between measures of those representations and the measures of L2 achievement used with each group. (Contains 11 tables, 2 figures, and 9 footnotes.)