The present article examines the effect of variable input on the acquisition of plural morphology in two varieties of Spanish: Chilean Spanish, where the plural marker is sometimes omitted due to a phonological process of syllable final /s/ lenition, and Mexican Spanish (of Mexico City), with no such lenition process. The goal of the study is to determine whether variable input for grammatical morphology affects the acquisition process. Does the ambiguous nature (sometimes present and sometimes absent) of a form affect acquisition? To address this question, Experiment 1 examines the production of the plural marker in Chilean- and Mexican Spanish-speaking children, and Experiments 2 and 3 examine children's use of plural and singular indefinite noun phrases in comprehension. The results indicate that variable input affects acquisition, with Chilean children taking longer to acquire the plural marker than Mexican children. (Contains 12 tables, 2 figures, and 23 footnotes.)