Aims. We derive the physical properties of a filament discovered in the dark cometary-shaped cloud L1251. Methods. Mapping observations in the NH3(1, 1) and (2, 2) inversion lines, encompassing 300 positions toward L1251, were performed with the Effelsberg 100-m telescope at a spatial resolution of 40″ and a spectral resolution of 0.045 km s-1. Results. The filament L1251A consists of three condensations (α, β, and γ) of elongated morphology, which are combined in a long and narrow structure covering a 38′ x 3′ angular range (∼3.3 pc x 0.3 pc). Comparing the kinematics with the more extended envelope (∼61′ x 33′) emitting in 13CO, we find that: (1) the angular velocity of the envelope around the horizontal axis E → W is ΩCOEW ≈ -2 × 10-14 rad s-1 (the line-of-sight velocity is more negative to the north); (2) approximately one half of the filament (combined α and β condensations) exhibits counter-rotation with ΩαβEW ≈ 2 × 10-14 rad s-1; (3) one third of the filament (the γ condensation) co-rotates with ΩγEW ≈ -2 × 10-14 rad s-1; (4) the central part of the filament between these two kinematically distinct regions does not show any rotation around this axis; (5) the whole filament revolves slowly around the vertical axis S → N with ΩtotSN ≈ 7 × 10-15 rad s-1. The opposite chirality (dextral and sinistral) of the αβ and γ condensations indicates magnetic field helicities of two types, negative and positive, which were most probably caused by dynamo mechanisms. We estimated the magnetic Reynolds number Rm ≳ 600 and the Rossby number R <1, which means that dynamo action is important.