Growth of Byssochlamys nivea in anaerobiosis and in pure culture on sterilized standing forage, or in ecological conditions very similar to ensilaging, is accompanied by the production of patulin and byssochlamic acid. In pure culture and in anaerobiosis (Trial E.ana), Byssochlamys nivea produces 42.5 p.p.m. of patulin on day 40 of storage and 34.25 p.p.m. of byssochlamic acid on day 82. In competition with other moulds (Trial E.ster.) or in presence of the complex microflora of silages (Trial E.f.), Byssochlamys nivea toxin production is irregular and less than found in Trial E.ana. A maximum of patulin is obtained on day 45 and day 82 of culture, respectively. Irregularity of the curves may express random competition between various micro-organisms or fixation of patulin on sulfhydryl radicals. Production kinetics of the two mycotoxins indicate that silage pollution risks will be high after three months of storage.