High Rydberg levels of ytterbium were studied in a beam of metastable atoms. Levels of the series 4f
14 6
snd were populated from the metastable 4f
14 6s 6p
3P
o level by means of the U.V. light of a frequency-doubled single-mode dye laser and detected using the field ionization technique. The critical ionization fields
Fc of the levels 4f
14 6
snd
3D
1 (24 ≤ n ≤ 57) were measured and found to obey the saddle point law (
Fc = (16
n∗4)
−1). The hyperfine structures of the odd isotopes were investigated. They are very peculiar due to the fact that the hyperfine interaction is of the same order of magnitude as (or even for the highest
n values, much more important than) the spin-orbit and electrostatic interactions. A theoretical account of the observed structures using the Slater-Condon parametric method is given. The relative influence of the excited (
nd) and of the non-excited (6s) optical electron could thus be studied. Furthermore the influence of a valence level perturbing only the odd isotopes components for
n = 26 was demonstrated.