[en] The distortion moment microwave spectrum of /sup 13/CH₄ has been studied with a conventional Stark modulated instrument having a sensitivity of approx.1 x 10/sup -11/ cm/sup -1/. Six pure rotational Q branch transitions have been observed between 8 and 20 GHz. With the assumption that the three octic tensor distortion constants L/sub T/, L/sub T/, and L/sub T/ have the same values as in /sup 12/CH₄, values have been obtained for the one quartic and two sextic tensor distortion constants. These are (in Hz) D/sub T/= (132 983.32 +- 0.48); H/sub T/=-(17.0026 +- 0.0018); H/sub T/= (11.1869 +- 0.0011). The errors given here are the standard deviations obtained from the least squares analysis. The justification for the assumption is presented, and estimates are given for the absolute errors in D/sub T/, H/sub T/, and H/sub T/, as well as for the isotopic shifts in L/sub T/, L/sub T/, and L/sub T/. The measured differences between the /sup 13/CH₄ and /sup 12/CH₄ values of D/sub T/, H/sub T/, and H/sub T/ are discussed with reference to recent theoretical calculations. A method is given of using these differences to calculate the tensor energy splittings of /sup 13/CH₄ from those previously published for /sup 12/CH₄