Two new methylpolyynes, CH(3)(C&tbond;C)(6)H (tridecahexayne) and CH(3)(C&tbond;C)(7)H (pentadecaheptayne), have been detected in a supersonic molecular beam by Fourier transform microwave spectroscopy. A total of 20 transitions of CH(3)(C&tbond;C)(6)H in the K = 0 and 1 rotational ladders and 8 transitions of CH(3)(C&tbond;C)(7)H in the K = 0 ladder were observed between 5 and 11 GHz, allowing for both molecules precise determination of rotational and centrifugal distortion constants. Like other long carbon chains of similar length, the rotational constants of the two new methylpolyynes are well predicted by extrapolation from the shorter members of the sequence; the addition of successive C(2) units evidently little perturbs the structure of the rest of the molecule. Centrifugal distortion is well described by the same semiclassical model that works well for the cyanopolyynes, one which treats a carbon chain as a thin elastic rod with a Young's modulus independent of length. Copyright 1999 Academic Press.