The hyperfine structures in the spectrum of Pb III, excited by an electrodeless discharge, have been studied by means of a 21 ft. concave grating. The hyperfine structure patterns lead to an I-value of [Formula: see text] and a g(I) factor of 1.2 for the odd isotope [Formula: see text], in agreement with the results of previous workers. The isotope shifts between the even isotopes [Formula: see text] and [Formula: see text] are in agreement with Breit's theoretical interpretation, and amount to 0.5 cm.−1 for each 6s electron. The fact that the deep terms in the spectrum of Pb III are rather completely known permits an attempt to be made at a careful comparison between experiment and theory.