This study is an experimental investigation of two ways to present seventy low English proficiency Japanese high school students with the multiple meanings of forty polysemous words over a ten-class period. In the control condition, the meanings of the words were presented in sentences and in the experimental condition, students were taught the underlying meaning of the polysemous words. The experimental treatment originates both in Nation's (1990) suggestion that a more systematic approach to the instruction of the semantic structure of polysemous words should be employed by drawing students' attention to the underlying meaning of those words and in cognitive linguists' explanation of the structure of polysemy (Lakoff, 1987). This study provides the first empirical support of Nation's proposal that teaching the underlying meaning of polysemous words is effective. The overall ANOVA results showed better retention of the target meanings by the experimental group ( p = .004), equal effects for different proficiency levels, and a statistically nonsignificant effect for guessing the unknown meanings of untaught words in context. The results suggest that (a) reference specification is a generally better strategy than sense selection for low proficiency EFL students, (b) the treatment led the participants to notice the meaning network of the target lexis as a result of decontexualization and to retain much of that lexis as they tried to establish new meaning networks with the word's underlying meaning as a unifying force, (c) explicit teaching accelerates the developmental shift from word association to concept mediation in the L2 mental lexicon (Kroll & Curley, 1988), and (d) explicit instruction works well for semantic mediational aspect of vocabulary acquisition. A principled way to present multiple meanings is proposed based on Nation's (2001) three principles of noticing, retrieval, and generative use. In sum, teachers should help learners become aware of the relationship between the core and peripheral meanings of polysemous words, preferably based on cognitive linguists' analyses of the metaphoric relations among words' meanings. (Atsushi, Nakahara)