PKS 0558‐504 is one of the few Narrow‐line Seyfert 1 galaxies that is radio‐loud. Here we present the first results of a monitoring campaign of PKS 0558‐504 carried out with RXTE for one year. The source shows large flux variability on timescales of days and weeks. The spectral variability is weak and apparently not correlated with the flux. In the time‐averaged energy spectrum there is no evidence for reprocessing features. The X‐ray properties suggest that PKS 0558‐504 is a peculiar object that appears to be different from both radio‐quiet and radio‐loud AGN, and might be a large‐scale analog of the Galactic black holes in the intermediate spectral state.