In vitro digestibility measures allow to predict the in vivo digestibility of roughage organic matter with a better accuracy than the proximate chemical analysis. In the present study a comparison was made using mixed diets fed to lactating dairy goats at various levels of intake and proportions of concentrates. Moreover, two kinds of concentrate feeds varying by their proportion of cell-wall constituents were studied. The in vitro digestibility technique applied to feedstuffs taken separately or combined in the same proportions as those of the ingested diet allowed to predict the in vivo digestibility of the whole diet with a higher accuracy than that of the proximate chemical analysis.