The present study investigated how the frequently used English word wear is understood by American students and Japanese college students studying English as a second language. The results of the two experiments revealed very different patterns between the native speakers and the L2 learners. The American students knew almost all of the senses of the word wear whether used in their concrete, prototypical senses or metaphorically extended senses and grouped the different senses into tightly cohesive clustres, which in turn comprised an orderly structured category of the word as a whole. In contrast, the Japanese students' understanding of the word was extremely impoverished, consisting only of the senses corresponding to the Japanese word “kiru”. The pattern obtained for the native speakers was consistent with a recently proposed theory treating word meaning as a structured category with single or multiple prototypes. The large difference observed between native speakers and Japanese students in understanding the meaning of a basic, frequent word such as wear points to the problem of traditional vocabulary instruction in second language classrooms, which exclusively relies on dictionary definitions.