The aim of this work was to determine occurrence of fungi from genus Fusarium and produced by them mycotoxins in spring barley cv. Progress cultivated in organic and conventional production systems after onion, carrot and cabbage. The study were carried out in the years 2006-2007. Macroscopic and mycological analysis of ears healthiness were performed during milk maturity of grain. Kernels of barley after grinding were analyzed on presence of mycotoxins: deoksynivalenol, 3-acetylodeoksynivalenol, 15-acetylodeoksynivalenol, T-2, HT-2 toxins, deoxyscirpenol, niwalenol, fuzarenon X , using gas chromatography coupled with ECD detector. The clearing of samples was made according the SPE method. In mycological analysis Fusarium poae was isolated most frequently from kernels. In the case of both systems in the year 2006 all analyzed samples included deoxynivalenol. Conventional barley was much less contaminated by all analyzed mycotoxins than organic. The smallest total content of mycotoxins was found in barley cultivated after onion: in conventional system it amounted 0,32 mg/kg and in organic system 10,0 mg/kg. The largest total content of mycotoxins was found in organic barley after cabbage and it amounted 12,64 mg/kg, and in conventional system after carrot, which amounted 8,42 mg/kg. In 2007 the content of mycotoxins in kernels of barley was lower in comparison with season 2006 as the result of weather conditions. In 2006 heavy rainfall occurred just before harvest, which caused development of Fusarium spp. on ears of barley and production of mycotoxins. In 2007 weather conditions were not favourable for development of fungi from genus Fusarium. In this year kernels of barley were less contaminated by mycotoxins in organic system of production than in conventional. Most beneficial forecrop for barley in aspect of mycotoxins reduction proved to be onion. More effective action of onion as forecrop in organic production can be related to larger biological activity of the plant in this production system.