The extent of vegetation used by free browsing goats on a rangeland is a practical measure to determine rangeland productivity and may be measured by the similarity indices between the botanical composition of diet and the rangeland. The aims of this study were to evaluate and compare, seasonally the similarity indices of forage species and goats diets under the Sonorant desert conditions. Samplings were carried out in summer and autumn of 2006, and winter 2007 in a 200-ha rangeland with an animal density of 0.13 to 0.36 individuals/ha. Twenty two fixed transects (30 m long) distributed randomly were used to measure the botanical composition and importance value (IV) for plant types by the line-intercept method. Identified species were classified as non legumes trees and shrubs (NLTS), legumes trees and shrubs (LTS), cacti, forbs, agaves and grasses. Five castrated Creole-Nubio goats  (29 kg of BW) provided with esophageal fistula were used to obtain diet samples and determine botanical composition of diets by microhistological analyses. Botanical composition of diet (A) and IV values (B) were used to calculate the similarity indices (SI) according to S = 2(W)(100)/A+B, being W the lower value between A and B. Sixty plant species were founded in the study area, but only 23 species were in diet of goats. Similarity indices indicated that NLTS were more utilized in all seasons than other plant types. The SI between seasons indicated a gradient of forage utilization in the following order (P NLTS (70.2) > LTS (39.1) > cactus (0.0) = grasses (0.0), in autumn; cactus (85.9) > NLTS (85.6) > forbs (77.1) > LTS (73.3) > grasses (6.9) and in winter LNTS (88.8) > LTS (88.0) > grasses (87.4) > forbs (70.8) > cactus (53.3). Goats utilized only 30% of the vegetation on rangeland in all seasons and showed light change in their forage utilization pattern because of the opportunistic feeding behavior of these animals; however they maintain a high utilization on shrubs and trees.