The extent of ruminal forage dry matter digestion is a function of rate of digestion and rate of passage. It is a complex process that is affected not only by the forage composition, but also by the retention time of forage particles in the different segments of the gastrointestinal tract (Noziere & Micalet-Doreau, 2000). A major factor affecting ruminal NDF digestibility of forage is the rate at which the forage passes from the rumen. This is a unique characteristic of individual forages or fibre sources. Daily intake of digestible organic matter is also closely correlated with ruminal retention time for forages and this relation applied to both temperate and tropical feeds. The objective of the present study was to determine the ruminal retention time (RRT) and indigestible dry matter fraction (IF) of various Iranian forages using in situ nylon bag procedure.