Examined parental speech data demonstrating that linguistic input to children does not contain sufficient information to support unaided learning of the pronoun "one." Examined 18-month-olds' interpretation of sentences with a "one" substitution. Found that 18-month-olds have command of the syntax of "one." Because syntactic knowledge could not have been gleaned exclusively from input, infants' mastery constitutes evidence for the contribution of innate structure in acquiring a grammar. (Author/KB)