This article treats an underwater isovelocity surface duct overlying a negative velocity gradient layer that requires two distinct families of normal modes to represent the acoustic field in the image‐interference region. Each family contains an infinite number of modes. The significance of the two families of modes at short ranges is demonstrated by comparing numerical results obtained from ray theory with results from normal‐mode theory. The contribution to the total normal‐mode solution by the second family of modes is a complicated function of the gradient below the surface channel, the range, and the receiver depth. Within the image‐interference region, the second family of modes becomes increasingly important as the range and gradient decrease and the receiver depth increases. However, beyond the image‐interference region, the second family of modes may be neglected without appreciable error. The properties of the various modal elements are investigated and some generalizations concerning the second family are derived.