Overseeding warm-season legumes may be a feasible alternative to decrease N fertilizer cost and enhance forage quality of bahiagrass (Paspalum notatum Flugge) pastures in Florida. The objective of this study was to evaluate the use of ‘Ubon’ stylosanthes (Stylosanthes guianensis var. vulgaris x var. pauciflora) on forage production and animal performance of crossbreed heifers (370 ± 25 kg) grazing bahiagrass pastures during the summer. The study was conducted in Ona, FL from July to October 2010. Treatments were overseeding stylosanthes (S), bahiagrass pastures fertilized with 60 kg N ha^-1 (N), or control (bahiagrass pasture with no N fertilization, C) in a completely randomized design with three replicates. Pastures (0.25 ha) were sampled to determine herbage mass (HM), height, light interception, and crude protein (CP) every 28 d. Pastures were continuously stocked using a fixed stocking rate with two heifers per experimental unit. There were no differences on botanical composition (P = 0.54, SE = 5.9) among months, with stylosanthes and bahiagrass proportions of 18 and 82%, respectively. Pastures fertilized with N had greater HM (P = 0.05, SE= 0.3, Mean = 4.3 Mg ha^-1) compared to C and S pastures (3.3 and 3.2 Mg ha^-1 respectively), however there were no differences on herbage allowance (P = 0.22, SE = 0.2, Mean = 2.4 kg DM kg^-1 liveweight) among treatments. Differences were found between canopy height of the N treatment (P = 0.10, SE = 1, Mean = 13.5 cm) compared to C and S (10.9 and 12.0 cm, respectively). Consequently, N pastures had greater light interception (P = 0.08, SE = 2.4, Mean = 76%) than C and S pastures (69 and 68%, respectively). There was a treatment x month interaction on herbage CP concentrations. The interaction occurred because CP concentrations decreased in greater magnitude in the S and N than in the C treatment during the experimental period. Average daily gain (P = 0.28, SE = 0.07, Mean = 0.03 kg d^-1), and blood urea nitrogen (P = 0.16, SE = 1.0, Mean = 11.4 mg dL^-1) were similar among treatments. The increased HM on N treatments may potentially allow greater stocking rates when compared to pastures overseeded with stylosanthes.