Three polymorphic forms and various salts of the acid (3,4 Dichloro - 1 - 2 - fluorobifenil - 4-yl) cyclopropane carboxylic acid and its use in the Prevention or treatment of Alzheimer's disease.Claim 1: a Crystalline Polymorph (Form I) acid (3,4 Dichloro - 1 - 2 - fluorobifenil - 4-yl) cyclopropane carboxylic acid wherein has a pattern of x-ray diffraction Powder comprising Peaks characteristic expressed in Angles q approximately 2 - 3: application : a Crystalline Polymorph (Form II) acid (1 - 34 - Dichloro - 2 - fluorobifenil - 4-yl) cyclopropane carboxylic acid wherein has a pattern of x-ray diffraction Powder comprising Peaks characteristic expressed in qtheta Angles approximately 2 - 5: Claim: a Crystalline Polymorph (form. Iii) acid (1 - 34 - Dichloro - 2 - fluorobifenil - 4-yl) cyclopropane carboxylic acid wherein has a pattern of x-ray diffraction Powder comprising Peaks characteristic expressed in Angles approximately 2 - Q: claim 7: a Salt of 1 - (3,4 Dichloro acid - 2 - fluorobifenil - 4-yl) cyclopropane Wherein the carboxylic acid is selected from the group consisting of salts of NA, K, CA,L - arginine, D - and L - arginine, iisina. Claim 8: a Pharmaceutical composition which comprises a substantially Pure Polymorph according to any one of claims 1 to 6 or a Salt according to claim 7, and a pharmaceutically acceptable vehicle.Tres formas polimórficas y varias sales del ácido 1-(3&rsquo；,4&rsquo；-dicloro-2-fluorobifenil-4-il)ciclopropano carboxílico, y su uso en la prevención o el tratamiento de la enfermedad de Alzheimer.Reivindicación 1: Un polimorfo cristalino (Forma I) del ácido 1-(3&rsquo；,4&rsquo；-dicloro-2-fluorobifenil-4-il)ciclopropano carboxílico caracterizado porque tiene un patrón de difracción de rayos X en polvo que comprende picos característicos expresados en ángulos 2-q a aproximadamente: Reivindicación 3: Un polimorfo cristalino (Forma II) del ácido 1-(3&rsquo；,4&rsquo；-dicloro-2-fluorobifenil-4-il)ciclopropano carboxílico caracterizado
