The isotopic structures of the Zn I resonance lines 2139 Å (4s21S0 − 4s4p1P1) and 3076 Å (4s21S0 − 4s4p3P1) excited in an atomic beam source have been resolved with a Fabry–Perot etalon. Shifts in cm.−1relative to the Zn64component are + 0.016 and + 0.033 in 2139 Å, and + 0.023 and + 0.046 in 3076 Å for Zn66and Zn68, respectively. When the normal mass shifts are subtracted, the residual shifts for Zn68are + 0.010 (2139 Å) and + 0.030 cm.−1(3076 Å). The field effect theory of isotope shift predicts equal shifts and the specific mass effect predicts unequal shifts for the1Pand3Plevels. Therefore the difference of these residuals is a specific mass effect. The difference has the sense predicted by the specific mass theory and its value, 0.020 cm.−1, is a large fraction of the observed shifts. Thus the specific mass effect must be taken into account before the field effect theory can be used to obtain nuclear properties from isotope shifts in intermediate elements.