An Augmented Relative Price Spread (ARPS) model is employed to explain recent changes in real US beef wholesale-retail (WR) and hence farm-retail (FR) marketing margins. It is found that the surge in retail market concentration in 1999 most likely increased retail market oligopsony power relative to wholesale oligopoly power, ultimately changing real US WR beef marketing margins. The finding that higher oligopsony retail market power relative to oligopoly wholesale market power in the US beef industry was most likely responsible for the changes in US WR marketing margins in 1999 is important because it provides an economic justification for policy makers to regulate anticompetitive conduct by beef retailers.