The measurements of the rotational spectrum of tertiary butyl isocyanide, (CH3)3CNC, were extended to 730 GHz, J = 124, and K = 90. The new mm-wave and sideband FIR-laser measurements were combined with previous results to yield accurate spectroscopic constants for the whole frequency region 0-730 GHz. In the highest frequency transitions a splitting in the K = 3 lines was observed, allowing the determination of the splitting constant h3. The known values of h3 for C3v-type molecules are compared and empirical rules for their behavior are derived and discussed. © 1993 Academic Press, Inc.