Plasticity in hexagonal close-packed zirconium is mainly controlled by the glide of dislocations with 1/3 1210 Burgers vectors. As these dislocations cannot accommodate deformation in the [0001] direction, twinning or glide of c + a dislocations, i.e. dislocations with 1/3 1213 Burgers vector, have to be activated. We have performed in situ straining experiments in a transmission electron microscope to study the glide of c + a dislocations in two different zirconium samples, pure zirconium and Zircaloy-4, at room temperature. These experiments show that c + a dislocations exclusively glide in first-order pyramidal planes with cross-slip being activated. A much stronger lattice friction is opposing the glide of c + a dislocations when their orientation corresponds to the a direction defined by the intersection of their glide plane with the basal plane. This results in long dislocations straightened along a which glide either viscously or jerkily. This a direction governs the motion of segments with other orientations, whose shape is merely driven by the minimization of the line tension. The friction due to solute atoms is also discussed.