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In organic field-effect transistors (OFETs), mobility is carrier density dependent and varies across the transistor channel. Because the photogenerated charge is constant as a function of bias, spatially resolved photocurrent measurement locally measures the product of channel field and mobility. This product can be used to derive the local mobility across the channel. We applied this approach to demonstrate that, as expected, the mobility in the linear operation region is relatively constant across the channel, while in the saturation region, it decreases from the source contact to the drain one due to the decreased density of carriers on the drain side.