The authors demonstrate the production of the B1 aflatoxin by means of a strain of the fungus Aspergillus flavus which was captured on cereal feed in spring 1975. Besides the production of the mycotoxin, this fungus strain showed a very quick and intensive growth and development, in comparison with the laboratory strains of Aspergillus flavus. The obtained and isolated aflatoxin B1 could be identified by chromatography and spectral photometry only when the fungus strain was cultivated on a suitable substrate where it showed the lowest producting of the distrubing fluorescent metabolism by-products. That finding led to the conclusion that in some cases it is necessary, besides the classically used procedure of the diagnosis of aflatoxicosis, to require also the isolation of the possibly present strain of Aspergillus flavus and--after its recultivation in pure cultures on suitable media--the determination of aflatoxin production connected with their physical and chemical identification.