Summary: This paper describes the results of an investigation of the flow over two cylinders in which the diameter of the upstream cylinder is half that of the downstream cylinder. Measurements were made of the surface pressure distribution around the downstream cylinder, and of the base pressures and Strouhal numbers of the two cylinders for separations between 1·32 D and 3·88 D, where D is the diameter of the downstream cylinder, and for angles of stagger between 0° and 45°.With the cylinders in tandem two different flows were found, and it is suggested that there is a step change between the flows at a critical separation.The behaviour of the measured quantities with a change of the angle of stagger is found, in general, to depend on whether the separation is less or greater than the critical separation. For each separation an angle of stagger is found at which the pressure distribution has a strong asymmetry over the rear of die cylinder. These pressure distributions are similar to each other, and they indicate the presence of an enhanced flow over the upper half of the cylinder. Also the lift force, for a given separation, then has its maximum value.It is found that for separations greater than the critical separation the Strouhal numbers of the vortex shedding from the two cylinders are identical over a considerable range in the angle of stagger.