Straight ducts of constant cross-section with sinusoidally curved walls may be found in plate heat exchangers as well as in packing structures used in mass transfer operations or in catalytic reactors. Frictional pressure drop, in the case of fully developed laminar flow, depends on the shape of the cross-section. It is characterized by a generalized form of Poiseuilles law : (xi . Re) = K. Two types of cross-section formed by two parallel sinusoidally corrugated walls, with the amplitude (a) over cap, the wave length Lambda and the distance 2 (a) over cap are regarded. The two sine walls may be either phase shifted by pi, so that the walls touch each other and form an arbitrary number of equal tubelike ducts, or they may be in phase, so that the duct is similar to the one formed by two parallel planes. In both limits the values of K were numerically calculated for a number of aspect ratios 2 (a) over cap/Lambda. The whole range of aspect ratios has been covered by fitting polynomials in order to allow for interpolation of the calculated values.