The possibility of predicting the in vivo organic matter digestibility of forage by estimating in vitro digestibility using either rumen fluid or cellulase as incubation media in the filter bag method was studied. The in vivo OM digestibility of 35 forage samples was determined on 6 rams and in vitro digestibility was determined by two methods: rumen fluid (in a Daisy^II Incubator)-neutral detergent (RF-ND) or cellulase solution (also in a Daisy incubator)-neutral detergent (C-ND). The coefficients of correlation between in vivo digestibility and that estimated by the RF-ND and C-ND methods were 0.62 and 0.69, respectively. Regression analysis shows that both tested methods have good potential for predicting in vivo OM digestibility.