Measurements of the mean and r.m.s. components of the swirl velocity have been obtained over the intake and compression strokes of a single-cylinder, four-stroke, diesel engine motored, with and without compression, at rotational speeds between 325 rev/min and 675 rev/min. During the intake stroke, the mean-velocity values show only a small dependence on whether compression is applied - the levels of r.m.s. velocity are however noticeably lower under compressing conditions. The swirl profiles are characterized by a region of high velocity near the cylinder wall, corresponding to the strong jet-action induced by the masked-valve arrangement, with additional peaks in the profiles, indicative of the inward spiralling motion of the flow. By bottom-dead-centre, the peaks are no longer in evidence and the swirl tends towards solid-body rotation in the central regions of the cylinder. After the intake valve closes ($\theta$ = 215$^\circ$), the apparent centre of rotation shows an abrupt displacement in the direction of the valves and, under compressing conditions, the centre of rotation gradually returns towards the cylinder axis, although pure solid-body rotation about this point is not achieved until the piston reaches top-dead-centre. The swirl velocities show a steady decrease until $\theta$ $\approx$ 300$^\circ$, when the effect of the flow being forced into the combustion bowl gives rise to increasing velocities. Without compression, the flow accelerates near the cylinder axis past mid-stroke, but the swirl velocities decrease rapidly as the piston approaches top-dead-centre and air escapes through the exhaust valve.