Fungal probiotics are gaining increasing acceptance as growth promoters that act via the rumen fermentation. Products have been described based on Saccharomyces cerevisiae and Aspergillus oryzac (Williams and Newbold, 1990). However not all species of Aspergillus or strains of S. cerevisiae exhibit probiotic properties (Tapia and llerrera-Saldana, 1989; Newbold, 1990). The selection of fungi with probiotic properties would be greatly facilitated by the development of a rapid method for assessing their efficacy. In the present study the effects of one of these products (Amaferm, based on Aspergillus oryzae; AO) on short term incubations invitro were compared with results obtained previously in longer-term in vitro fermentations (Rusitec) and with in vivo measurements.