Abstract We report the detection of molecular CO(1–0) gas in F00183-7111, one of the most extreme ultraluminous infrared galaxies (ULIRGs) known, with the Australia Telescope Compact Array. We measure a redshift of 0.3292 for F00183-7111 from the CO(1–0) line and estimate the mass of the molecular gas in 00183 to be 1 × 1010 M⊙. We find that F00183-7111 is predominately powered by the active galactic nucleus (AGN) and only ∼14 per cent of the total luminosity is contributed by star formation (SFR ∼220 M⊙ yr−1). We also present an optical image of F00183-7111, which shows an extension to the east. We searched for star formation in this extension using radio continuum observations but do not detect any. This suggests that the star formation is likely to be predominately nuclear. These observations provide additional support for a model in which the radio emission from ULIRGs is powered by an intense burst of star formation and by a radio-loud AGN embedded in its nucleus, both triggered by a merger of gas-rich galaxies.