The mobility mu of solution-processed organic semiconductors has improved markedly(1,2) to room-temperature values of 1-5 cm(2) V(-1) s(-1). In spite of their growing technological importance(3), the fundamental open question remains whether charges are localized onto individual molecules or exhibit extended-state band conduction like those in inorganic semiconductors(4). The high bulk mobility of 100 cm(2) V(-1) s(-1) at 10 K of some molecular single crystals(5) provides clear evidence that extended-state conduction is possible in van-der-Waals-bonded solids at low temperatures. However, the nature of conduction at room temperature with mobilities close to the Ioffe-Regel limit remains controversial(6). Here we investigate the origin of an apparent 'band-like', negative temperature coefficient of the mobility (d mu/dT < 0) in spin-coated films of 6,13-bis(triisopropylsilylethynyl)-pentacene. We use optical spectroscopy of gate-induced charge carriers to show that, at low temperature and small lateral electric field, charges become localized onto individual molecules in shallow trap states, but that a moderate lateral electric field is able to detrap them resulting in highly nonlinear, low-temperature transport. The negative temperature coefficient of the mobility at high fields is not due to extended-state conduction but to localized transport limited by thermal lattice fluctuations.