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The microwave spectrum of $\beta$-methoxypropiontrile has been investigated in the region 12-40 GHz. The study has revealed the presence of only one rotational isomer, the all-trans form (Cs symmetry). This is in distinct contrast to the results for the homologous compound, methoxy-$acetonitrile,1$ which was found to be predominantly in the gauche conformation (methyl gauche to cyanide substituent). The stability of the title compound is interpreted in terms of an attractive dipole-dipole interaction. From an analysis of the dominant a type R-branch spectrum, the ground state rotational constants are (in MHZ ) $A=17800\pm 50,B=1425.53\pm 0.01,C=1353.89\pm 0.01$. Several vibrationally excited states have also been observed and will be $discussed.1$R. Kewley, Can. J. Chem. 52, 509 (1974).