We propose to carry out multiwavelength photometry at 4-200 mu of a complete all-sky sample of extreme ultraluminous IRAS galaxies, with log L(60) > 12.5, from the PSCz redshift survey. The discovery of ultraluminous infrared galaxies was one of the major results of the IRAS mission. Subsequent study of these galaxies has been hampered by lack of access to most of the infrared waveband. Detailed radiative transfer models for starbursts and AGN contributions to the far infrared continuum are porrly constrained by the IRAS fluxes. In most cases only 60 and 100 mu fluxes were detected by IRAS. Although there is a concensus that interactions and mergers play a major role in generating ultraluminous infrared galaxies, there is major controversy about whether massive starbursts or shrouded quasars are the principal contributors to the emission. Detailed spectral energy distributions over a wide spectral range will be a powerful tool in resolving the controversy. Study of a complete sample is essential for statistical and quantitative conclusions to be reached. Only a few of these extreme ultraluminous galaxies have been studied with ISO (the PSCz survey has only recently been completed). They probably represent extremely violent starbursts, but ISO photometry is capable of detecting dust emission at 5-20 mu from dust-shrouded AGN if these are present.