Ten spectral lines of the symmetric-top molecule methylcyanodiacetylene (CH3C5N) have been detected with the 100 m Green Bank Telescope (GBT) toward the Taurus molecular cloud TMC-1. Both K=0 and 1 components of the 12,K-11,K, 13,K-12,K, 14,K-13,K, 15,K-14,K, and 16,K-15,K transitions were observed. Consistent with ~10 K kinetic temperature for the TMC-1 dark dust cloud, no higher K-components were detected. The CH3C5N spectra confirm an earlier identification report based on data obtained with the MPIfR 100 m telescope by Snyder and colleagues over 20 years ago. Methyl cyanide (CH3CN), methylcyanoacetylene (CH3C3N), and methylcyanodiacetylene all are found in TMC-1 in decreasing relative abundance, suggesting that simple carbon addition on grains may be possible even in dark clouds. However, the correct formation mechanism for CH3C5N is still not known to within several orders of magnitude.