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We present a study of the HI distribution and dynamics of the nearby spiralgalaxy NGC5055 based on observations with the Westerbork Synthesis RadioTelescope. The gaseous disk of NGC5055 extends out to about 40 kpc, equal to3.5 R_25, and shows a pronounced warp that starts at the end of the brightoptical disk (R_25= 11.6 kpc). This very extended warp has large-scalesymmetry, which along with the rotation period of its outer parts (~1.5 Gyr at40 kpc), suggests a long-lived phenomenon. The rotation curve rises steeply inthe central parts up to the maximum velocity (v_max ~ 206 km/s). Beyond thebright stellar disk (R_25), it shows a decline of about 25 km/s and thenremains flat out to the last measured point. The standard analysis withluminous and dark matter components shows the dynamical importance of the disk.The best fit to the rotation curve is obtained with a ``maximum disk''. Lesssatisfactory fits with lighter disks help to set a firm lower limit of 1.4 tothe mass-to-light ratio in F band of the disk. Such a ``minimum disk''contributes about 60% of the observed maximum rotational velocity. NGC5055shows remarkable overall regularity and symmetry. A mild lopsidedness isnoticeable, however, both in the distribution and kinematics of the gas. Thetilted ring analysis of the velocity field led us to adopt different values forthe kinematical centre and for the systemic velocity for the inner and theouter parts of the system. This has produced a remarkable result: thekinematical and geometrical asymmetries disappear, both at the same time. Theseresults point at two different dynamical regimes: an inner region dominated bythe stellar disk and an outer one, dominated by a dark matter halo offset withrespect to the disk.

$\ion{H}{i}$ study of the warped spiral galaxy NGC 5055: a disk/dark matter halo offset?