Carrier mobility measurements were performed in the regioregular poly(3-hexylthiophene) (P3HT) and regioregular poly(3-octylthiophene) (P3OT) samples by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. The samples were produced by the drop-casting and spin-coating methods on the Indium-Tin-Oxide (ITO) substrate and provided with Al contact at the top. We had observed the "negative" mobility dependence at low electric fields, i.e., the mobility used to increase with decreasing electric field. To our knowledge this is the first time, when such behaviour was confirmed by the CELIV method in P3OT samples, though it was reported earlier by the Time-of-Flight (TOF) in disordered materials.