Infants across cultures need to identify the characteristics of their native languages in order to become competent speakers. The means by which Spanish-speaking children learn to produce number-gender linguistic markers has not been sufficiently investigated. Thirty-eight three-year-olds were tested in Berko-like production tasks, in which they were asked to pluralize or singularize familiar and novel words, with controls for allomorph, number of syllables, and word familiarity. Children found it easier to pluralize and singularize words with the allomorph /-s/ than those requiring /-es/, independent of their familiarity or syllable length. Children also produced a wide variety of noun phrases in which they tended to mark number information in more than one element. These data suggest that Spanish-speaking children's inflectional abilities are mainly influenced by phonological features such as word-endings and not, as previously reported, by the familiarity of the word or syllable length.