The rotational structure of the vibrationless S-1<--S-0 transition of pentacene has been investigated using a strongly collimated seeded supersonic argon beam. Because single rotational lines could not be completely resolved, a band contour analysis was performed. The rotational constants of the electronic ground state X (1)A(1g) were found to be under the asymmetric rotor approximation A " = 1320.6(9), B "=117.97(9), C "=108.28(15) MHz, whereas the differences to the first electronic excited state A B-1(2u) are Delta A =A＇ - A " = 13.2(3), Delta B = - 0.764(45), Delta C = - 0.54(6) MHz. A new value of the band origin was determined to be v(00) = 18 648.996(4) cm(-1) and the band type was confirmed to be of type b as proposed by symmetry arguments. Good agreement between observed and calculated spectra was obtained assuming planarity in both ground and excited state. From the fit procedure a rotational temperature of about 7 K was deduced. The nuclear statistical weights of the electronic ground state are reported.