Pentacene is among the most widely employed semiconductors for organic thin-film transistors (TFTs). The main reason is its large field-effect mobility (∼1 cm2/Ns) which results from the relatively large overlap of the delocalized molecular orbitals in the (001) lattice plane of the pentacene thin-film phase [1-4]. But pentacene molecules are easily oxidized at the 6 and 13 positions, and since the oxidation changes the electronic structure of the molecules, the mobility of pentacene TFTs degrades rapidly during air exposure [5,6]. Yamamoto et al. have recently synthesized a six-ring fused heteroarene, dinaphtho-[2,3-b:2',3'-t]thieno[3,2-b]thiophene (DNTT), which has a crystal structure and thin-film morphology similar to those of pentacene, but is less susceptible to oxidation . As a result of the favorable crystal structure and morphology, the mobility in DNTT is similar to that in pentacene, while the greater oxidation resistance affords better air stability of DNTT transistors compared with pentacene devices. Here we report on the static and dynamic performance and on the stability ofDNTT TFTs on flexible polyethylene naphthalate (PEN) substrates.