In this study 6 ruminally cannulated Nellore steers (407 ± 24 kg BW) were used in a 6 × 6 Latin square design to evaluate 5 levels of whole cottonseed (0, 8, 16, 24 and 32% DM basis respectively treatments WC0; WC8; WC16; WC24 and WC32) plus a negative control diet – WCO + soybean oil (treatment SO; to reach the WC0 diet at the same fat content of WC32). Isonitrogenous (14% CP) diets containing 20% roughage 80% concentrate were formulated to provide the same amount of ruminally degradable protein. The 26-d experimental periods consisted 21-d for adaptation followed by 5-d for collection. Increasing WC linearly decreased DMI (P < 0.01; 7.9; 7.21; 6.77; 6.06; 6.18 kg/d respectively for WC0; WC8; WC16; WC24 and WC32). No differences were observed between S0 and WC0 (P = 0.23; 7.17 vs 7.90 kg/d respectively) nether between S0 and WC32 (P = 0.08; 7.17 vs 6.18 kg respectively). Increasing WC increased quadratically the pH (P < 0.001; 6.24; 6.44; 6.54; 6.65; 6.56 respectively for WC0; WC8; WC16; WC24 and WC32). The SO inclusion increased the pH compared with WC0 diet (P < 0.05; 6.37 vs 6.24 respectively). Even with the same fat content WC32 diet presented higher pH than SO diet (P < 0.001; 6.56 vs 6.37 respectively). Total volatile fatty acids (VFA) linearly decreased with WC inclusion (P < 0.001; 107; 94.4; 87.0; 79.8 and 79.0 mM respectively for WC0; WC8; WC16; WC24 and WC32). The SO inclusion did not change total VFA compared with WC0 diet (P = 0.22; 102 vs 107 mM respectively). The SO inclusion increased the total VFA compared with WC32 diet (P < 0.001; 102 vs 79 mM respectively). Acetate:propionate (A:P) ratio linearly decreased with WC inclusion (P < 0.001; 2.60; 2.62; 2.22; 2.23; 1.88 respectively for WC0; WC8; WC16; WC24 and WC32). The SO inclusion did not change the A:P ratio compared with WC0 diet (P = 0.73; 2.66 vs 2.60 respectively) but compared with WC32 the SO increased the A:P ratio (P < 0.005; 2.66 vs 1.88 respectively). Intake and rumen fermentation parameters were negatively affected by the levels of WC inclusion in the diets but the main reasons for that not seems only related with the increased fat content.