Using ultrasonic spotwelds, two series of overlap joints were prepared with aluminum alloys and tested in tension. In the first series between 4x12x0.063 in.-thick panels, it is shown that on a one-for-one basis ultrasonic welds produce joints superior to those made with Vi-in. steel fasteners. In the second series on 7075T6 and 2024-T3 alclad sheets 12 x 30 x 0.063 in. thick, joints were made with different numbers and arrays of spots. Joints based on row separations of 2 in. and pitches on the order of /4 in. are shown to be optimum and able to develop sheet stresses in excess of the yield strengths at failure. At pitch distances greater than A in., the higher individual spot loads can lead to shear and nugget failures as well as lower sheet stresses at failure. With a more dense population than at optimum pitch, there is a distinct reduction of sheet stress at failure. This is due, apparently, to restricted yield strain redistributions along the edges of the close packed rows of spots.