The function of the ‘third compartment’ of the ruminant forestomach, the omasum, has been debated for a long time. To date, it is assumed that its major function is fluid reabsorption. In order to investigate differences in this organ between ruminant feeding types, we first compared macroscopic measurements of the omasa of free-ranging muskoxen Ovibos moschatus [n=6, mean body mass (BM) 207 kg, range 180–221], a grazer, and free-ranging moose Alces alces (n=11, mean BM 291 kg, range 144–418), a strict browser. Despite the similar BM range, omasa of muskoxen contained more ingesta, had a higher empty organ weight, had more third- and fourth-order laminae, and represented a higher proportion of the total forestomach weight. In particular, the surface area of the omasal leaves – the area available for fluid absorption – was significantly larger in muskoxen (10 933±940 cm2) than in moose (2228±885 cm2). In order to test whether the difference in available surface area is a true functional correlate of feeding type, additional data on the omasal laminar surface area were generated for 83 individuals of 19 species. These data were supplemented with data on 13 additional species from the literature. The percentage of grass (%grass) in the natural diet was used to characterize the feeding type; the phylogenetic tree used for a controlled statistical evaluation was entirely based on mitochondrial DNA information. Regardless of phylogenetic control in the statistical treatment, there was a significant positive correlation of both BM and %grass in the natural diet with omasal laminar surface area. The data suggest that certain ruminant species that ingest more grass have larger omasal leaf surface areas, possibly indicating a higher need for water reabsorption distal to the ruminoreticulum, which could be explained as a consequence of the more distinct rumen contents stratification in these species.