The national four-firm concentration ratio in the lamb slaughtering and processing industry increased from 55 percent in 1980 to 70 percent in 1992. The effect of increasing lamb packer concentration on lamb marketing margins is examined. A relative price spread (RPS) model for farm-to-wholesale and wholesale-to-retail marketing margins was estimated using three-stage least squares (3SLS). The 3SLS results indicate that increased lamb packer concentration has had relatively small, positive effects on lamb marketing margins.