We performed an ergonomic assessment of a passive upper body exoskeleton at on-site job tasks, using surface electromyography (EMG) to quantify muscular demands at the shoulder. Six workers were assessed during cab assembly, hydraulic assembly, parts painting, parts hanging, and frame welding tasks, both with and without an exoskeleton. Workers were evaluated during consecutive job tasks, at beginning and at end of a shift. EMG amplitudes were measured and compared to recommended threshold limit values (TLVs) to determine fatigue risk values. Wearing the exoskeleton significantly reduced anterior deltoid EMG amplitudes and fatigue risk values during the consecutive job cycles, with no significant effects on the biceps brachii, superior trapezius, or lumbar erector spinae. Reduced EMG amplitudes were found at five of the six job tasks, and in three of the tasks fatigue risk values were reduced from above the TLV without the exoskeleton to below the TLV with the exoskeleton. Workers indicated that they would recommend the exoskeleton to others who perform overhead job tasks, but expressed concern that it might interfere with tasks that involve tight spaces, sitting, and bending over. We suggest evaluating specific job tasks when making decisions about exoskeleton usage.