  This paper presents the Hubble diagram (K-z relation) for FIRST (Faint Images
of the Radio Sky at 20 cm) radio sources identified in the Bootes and Cetus
fields. The correlation between the K magnitude of the FIRST-NDWFS sample and
the photometric redshifts found to be linear. The dispersion about the best fit
line is given by 1.53 for the whole sample and 0.75 at z>1. The paper also
presents a composite K-z diagram of FIRST radio sources and low-frequency
selected radio samples with progressively fainter flux-density limits (3CRR,
6C, 7CRS and the EIS-NVSS sample). The majority of FIRST radio sources lie
fainter than the no evolution curve (3L* galaxies) probably highlighting the
fact that the galaxy luminosity is correlated with the radio power.
