The present invention relates to crystalline forms of ethyl[3-[2-chloro-4-fluoro-5-(1- methyl-6-trifluoromethyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-3-yl)phenoxy]-2-pyridyl- oxy]acetate (CAS 353292-31-6), herein after also referred to as "compound of formula (I)". The invention also relates to a process for the production of these crystalline forms, formulations for plant protection and herbicidal compositions which contain one of these crystalline forms.