The existence of the medium-scale waves (wavenumbers 4-6), especially wavenumber 5, is a “ubiquitous” feature of the Southern -Hemisphere general circulation during the summer season. The data generated by the FGGE III-b analyses of the European Center for Medium Range Weather Forecasts were used to explore the role played by this wave regime on three dynamic aspects of the Southern-Hemisphere general circulation: eddy transport property, vacillation of atmospheric energetics, and downstream development. The medium-scale waves contribute about a half of the total eddy transport of sensible heat and momentum in the Southern-Hemisphere summer general circulation. It is inferred from the contrast between the sensible heat transport by this wave regime in the upper and lower levels along the Pacific storm track that the sea surface temperature may affect the development of medium-scale waves. The momentum transport by this wave regime maintains the jet stream. The standard deviations of eddy energy variables are largely attributed to the medium-scale waves. If the atmospheric energetics of the summer Southern Hemisphere exhibit a vacillation, then the medium-scale wave should play the most important role, compared to other scales of atmospheric motions. Time series analysis of various energy variables prove this conjecture. The medium-scale waves showed several cases of downstream development during the 1978/79 summer. One case was used to illustrate physical processes involved over various phases of the entire life cycle of downstream development. It was shown that the downstream development of medium-scale waves was amplified by the baroclinic process during the developing stage and maintained by the barotropic process during the decaying stage.