The present invention relates to co-crystals of pyrimethanil and dithianon, which, in a X-ray powder diffractogram at 25° C. show at least three of the following reflexes: 2&thetas；=7.46±0.20 2&thetas；=9.98±0.20 2&thetas；=13.28±0.20 2&thetas；=23.09±0.20 2&thetas；=24.38±0.20 2&thetas；=27.01±0.20 a process for their preparation and their use for the preparation of compositions for crop protection.
