Using an in vitro rumen fermentation method previously described (Donefer et al., 1960) 12-hour cellulose digestion coefficients were determined for samples of dry forages for which Nutritive Value Indices had been obtained by feeding the forages in the chopped and/or ground form to sheep. Separate regression analyses were required for chopped and ground forages since the NVI of a forage may vary with the physical form in which it is fed, although the in vitro cellulose digestion of that forage, based in all cases on finely ground samples, provides a single value. The 12-hour in vitro cellulose digestion (X) was found to be highly correlated with the NVI of 26 forages fed chopped (r=0.91) and 16 forages fed ground (r=0.87). Since the regression coefficients for both physical forms were not found to be heterogeneous, they were pooled to give a single value. Prediction equations for the NVI (Y) of chopped and ground forages are, respectively. The latter equation can also be expressed as Ŷ=-3.5+1.23X+10.9 to illustrate the observed increase of 10.9 NVI units as a result of the grinding of the forage.