Tristable electrical conductivity switching and non-volatile memory effects are demonstrated in a conjugated copolymer of poly(2,6-diphenyl-4-((9-ethyl)-9H -carbazole)-pyridinyl-alt-2,7-(9,9-didodecyl)-9H -fluorenyl) (PPCzPF). The indium-tin oxide (ITO)/PPCzPF/Al device can be switched from the low-conductivity (off) state to the first high-conductivity (on-1) state at 1.8 V, with an on/off current ratio of approximately 100. The device can be further switched from the on-1 state to the next higher conductivity (on-2) state at 2.4 V, with an on-2/on-1 current ratio of approximately 20. All the three conductivity states are accessible, stable and nonerasable. The tri-level conductance switching can be explained in terms of field-induced conformational ordering of the polymer chains and enhanced charge-transfer interaction at the PPCzPF/ITO interface.