Laser Stark spectroscopy has been performed for the first time on free pentacene molecules seeded in a highly collimated supersonic argon beam. The Stark effect of the vibrationless S-1<--S-0 transition of pentacene at 18 649 cm(-1) was investigated in a static homogeneous electric field up to 81 kV/cm. Within the unresolved rotational structure, an unexpected large quadratic Stark effect was observed. An analysis of the shifts of sub-band heads and characteristic structures was performed by simulating the complete Stark spectrum using second-order perturbation theory. Surprisingly large values for the differences of the molecular polarizability between the electronic ground state X (1)A(1g) and the excited A B-1(2u) state were found: Delta(P) over bar =1/3(DeltaP(aa)+DeltaP(bb)+DeltaP(cc))=9.1(3)x10(2) Angstrom(3), DeltaP(cc)=P-cc'-P-cc"=13.7(8)x10(2) Angstrom(3), DeltaP(xx)=-1.4(8)x10(2) Angstrom(3), DeltaP(yy)=15.0(7)x10(2) Angstrom(3) (x=a, y=b, or vice versa). Attributing the in-plane axes uniquely and the determination of the ground state anisotropy was not possible. It is assumed that the large polarizability of the excited state is dominated by contributions of low-frequency vibrational levels. Indications for deviation from second-order perturbation theory are observed. (C) 2003 American Institute of Physics.