The purpose of this study was to compare muscle activation of the anterior deltoid, medial deltoid, and pectoralis major during a Smith machine and free weight bench press at lower (70% 1RM) and higher (90% 1RM) intensities. Fourteen experienced (age, 19.9 ± 2.1 years; height, 176.3 ± 7.5 cm; mass, 88.5 ± 19.4 kg) and twelve inexperienced (age, 20.5 ± 2.1 years; height, 179.8 ± 8.0 cm; mass, 75.5 ± 10.4 kg) men completed two testing sessions. Investigators counterbalanced the order of conditions (free weight, Smith machine) and randomized the order of loads (70 and 90% 1RM) to control for biasing of order for each participant. The sessions consisted of determining each subject's 1RM on either the Smith Machine or free weight bench press followed by two repetitions at 70% 1RM and two repetitions at 90% 1RM on the tested mode. One week later, subjects completed the same protocol for the other mode. Surface EMG electrodes were placed superficial to the anterior deltoid, medial deltoid and pectoralis major muscles prior to data collection. Activation of the medial deltoid was significantly greater on the free weight bench press compared to the Smith machine bench press, regardless of load or experience level. There was no difference in the activation of the anterior deltoid or pectoralis major between modes or experience level. Muscle activation was significantly greater at the 90% 1RM load compared to the 70% 1RM load. The results suggest that the instability caused by the free weight bench press necessitates a greater response by the medial deltoid as a stabilizer of the humerus in the glenohumeral joint. The relative constancy in muscle activation of the anterior deltoid and pectoralis major between modes suggests that these two muscles do not play as large a role in stabilizing the shoulder on free weight bench. Alternatively, it may be that the increased stability offered by the Smith machine decreases the need for the anterior deltoid and pectoralis major to stabilize, allowing them to produce more force. The bench press is a common exercise performed by both athletes and recreational lifters and prescribed by trainers. The results of this study help both the practitioner and trainer in providing more information about the differences in between performing the bench press exercise on a Smith machine and on a free weight bench. Specifically, these results suggest that the free weight bench press may lead to an increased requirement for stabilization from muscles such as the medial deltoid.