Measurements of the hyperfine structure of the line $\lambda 5895$ of Sb II by the use of a Fabry-Perot interferometer have shown that there are deviations from the interval rule in the case of the isotope with mass number 123. The hyperfine structure intervals of the ${}_{}{}^3{}_{}{}^{}D_1^{}{}_{}{}^{}$ level of ${\mathrm{Sb}}^{121}$ (I=5/2) are found to be 0.6801 and 0.4832 ${\mathrm{cm}}^{-1\mathrm{}}$. The corresponding intervals in ${\mathrm{Sb}}^{123}$ (I=7/2) are 0.4814 and 0.3603 ${\mathrm{cm}}^{-1\mathrm{}}$. The error in these measurements is of the order of 0.001 cm. The observed ratio of the two separations gives 1.407 and 1.336 for the light and heavy isotope, respectively, whereas the corresponding ratios on the basis of the interval rule are 1.400 and 1.286. The deviation in the case of the light isotope is within the experimental error. The deviation for the heavy isotope, however, is a real effect which cannot be accounted for on the basis of perturbing effects of neighboring levels and must therefore be ascribed to the presence of an electric quadrupole moment of the nucleus of ${\mathrm{Sb}}^{123}$.

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