Disclosed is a process for preparing solid fipronil comprising at least 85 percent by weight of the crystalline modification I of fipronil being present in the monoclinic system having the centrosymmetric space group C2/c comprising the steps of: step i) preparing a solution of a solid form of fipronil being different from the crystalline modification I in a solvent S comprising at least one acetonitrile, dimethylsulfoxide, or at least one benzene derivative； step ii) effecting crystallization of fipronil； and step iii) isolating the resulting precipitate wherein, in case the solvent S is pure acetonitrile, the crystallization step ii) is conducted at temperatures below 40 degrees Celsius and in case the solvent S is pure dimethylsulfoxide, the crystallization step ii) is conducted at temperatures below 80 degrees Celsius.
