Biosensors based on a field-effect transistor platform allow continuous monitoring of biologically active species with high sensitivity due to the amplification capability of detected signals. To date, a large number of sensors for biogenic substances have used high-cost enzyme immobilization methods. Here, highly sensitive organic field-effect transistor (OFET)-based sensors functionalized with synthetic receptors are reported that can selectively detect acetylcholine (ACh(+)), a critical ion related to the delivery of neural stimulation. A cucurbit[6]uril (CB[6]) derivative, perallyloxyCB[6] ((allyloxy)(12)CB[6], AOCB[6]), which is soluble in methanol but insoluble in water, has been solution-deposited as a selective sensing layer onto a water-stable p-channel semiconductor, 5,5-bis-(7-dodecyl-9H-fluoren-2-yl)-2,2-bithiophene layer. The OFET-based sensors exhibit a detection limit down to 1 x 10(-12) m of ACh(+), which is six orders of magnitude lower than that of ion-selective electrode-based sensors. Moreover, these OFET-based sensors show highly selective discrimination of ACh(+) over choline (Ch(+)). The findings demonstrate a viable method for the fabrication of OFET-based biosensors with high sensitivity and selectivity, and allow for practical applications of OFETs as high-performance sensors for biogenic substances.