The relativistic equation of motion with a resistance force of Newtonian type cannot be simply given byFr = m0dvr/dτ+λvr. It is shown that the relativistic form corresponding to the classical equation of motion with a resistance force proportional to thenth power of the velocity is given byFr = m0dvr/dτ+λ(zvr − ur)[(zvs − us)(zvs − us)](n−1)/2, whereur, is the four-velocity of the fluid andz = −vsu3/c2. This equation may be considered as “physical,” because it can be reduced to particular exact (i.e., relativistic) equations describing real motions. Other mathematically correct (i.e., relativistically covariant) equations which reduce, for low velocities, to the same classical expression, cannot.