|Abstract:||Much recent work in phonology is concerned with evaluating contrasts among sets of forms (e.g. Flemming 1995, 1996; Ito and Mester 2003; Lubowicz 2003; Padgett 2003, 2004). This paper extends contrast preservation to syntax, where it can be used to explain anti-ambiguity phenomena. One such syntactic phenomenon is found in Japanese. Scrambling is generally tolerated in Japanese; however, when a subject and an object are morphologically identical (i.e. not distinguished by case morphology), scrambling is blocked. If scrambling were allowed in such a sentence, ambiguity would arise as to the subject of the sentence: 'Taroo-ga Hanako-ga kowai' could be the surface form of either an unscrambled structure 'Taroo-ga Hanako-ga kowai' (Taroo is afraid of Hanako) or a scrambled structure *'Taroo-ga(i) Hanako-ga t(i) kowai' (Hanako is afraid of Taroo). The ungrammaticality of the latter scrambled structure prevents subject-related ambiguity. I argue that this word order freezing (which occurs in German, Hindi, and Korean as well as Japanese: Lee 2001) is motivated by a PF constraint which demands contrast between forms with different subjects.