Nomadic pastoralism has been the traditional method of utilizing grazing resources in arid and semi-arid regions of Africa. However, increased sedentarization accompanied by growing human and animal populations during the past two decades is thought to be accelerating the desertification process, or desert expansion. The specific interactions of the grazing animal with this process has been speculated upon but not studied in detail. A comparative study was initiated during the spring grazing season of 1974 to determine sheep and goat nutritional and production responses, as well as patterns of vegetative selection and utilization under the pastoral system currently employed in the Pre-Saharan region of southern Tunisia. The study site was located on a sandy soil dominated by the perennial shrub, Rhanterium suaveolens. Annual herbs were co-dominants of this community in early spring. Four grazing treatments were employed during a month-long grazing season. These included; sheep grazing alone, goats grazing alone, sheep grazing with goats, and goats grazing with sheep. Stocking rates (1.9 sheep or goats per hectare per month) were comparable to those locally employed. Dietary composition was determined for randomly selected animals by a modified bite-count method, Forage intake was determined by the equation, I = F/1-D where I represented intake rate, F represented fecal output as determined from collections using standard fecal bags, and D represented digestibility of composite diets as determined by in vitro techniques. Animals were weighed weekly. Diets of all treatment groups, except goats in the mixed herd, consisted primarily of annuals during the first week. During Week Two there was a gradual shift to perennials and by the third week, all treatment groups selected primarily perennial species for their diets, Perennials comprised over 90% of the diets, except for sheep in the mixed herd, by the end of the fourth week. Also, by Week Four, Rhanterium comprised 71-92% of the diets. Estimates of forage quality indicated a declining trend in nutritional value of the forage over the grazing period. Dry matter consumption, digestibility of the diets, consumption of apparent digestible energy, dietary crude protein and apparent digestible protein all decreased from Week One to Week Four. These changes were probably attributable to a combination of factors including a decrease in plant species availability due to grazing, maturation of the remaining vegetation and a dietary shift from annuals to perennials. Young animals gained weight at generally increasing rates throughout the grazing trial. Adult animals gained weight after the initial week but their rate of gain indicated a leveling off or even a decrease by the fourth week, probably in response to declining forage quality. Animals in the mixed herd traveled farther during daily grazing periods than either of the single species herds. Goats grazing alone traveled farther than sheep grazing alone. Goats in the mixed herd may have influenced the sheep in that herd to travel more than sheep grazing alone.