Second language acquisition studies can contribute to the body of research on the influence of language on thought by examining cognitive change as a result of second language learning. We conducted a longitudinal study that examined how the acquisition of Spanish grammatical gender influences categorization in native English-speaking adults. We asked whether learning the grammatical gender of Spanish affects adult native English speakers' attribution of gender to inanimate objects. College students enrolled in beginning Spanish participated in two tasks repeatedly (four times) throughout one academic year. One task examined their acquisition of grammatical gender. The other examined their categorization of inanimate objects. We began to observe changes in participants' grammatical gender acquisition and in categorization after ten weeks of Spanish instruction. Results indicate that learning a second language as an adult can change the way one categorizes objects. However, the effect of Spanish grammatical gender was more limited in Spanish learners than in native Spanish speakers; it was not observed for all kinds of objects nor did it increase with learners' proficiency, suggesting that adults learning Spanish reach a plateau beyond which changes in categorization do not occur.