To study whether patulin diffuses in front of the pathogen or vice versa, we have inserted a constitutively expressed gfp-construct into Penicillium expansum. Spores and hyphal growth of the transformant IBT21525gfp4 in apple tissue was visualized directly by fluorescence mocroscopy. Using HPLC/UV, patulin was detected in apparently sound tissue more than 2 cm from the edge of lesions caused by P. expansum wildtypes.