A spatially developing countercurrent mixing layer was established experimentally by applying suction to the periphery of an axisymmetric jet. A laminar mixing region was studied in detail for a velocity ratio R = AU/2Ū between 1 and 1.5, where ΔU describes the intensity of the shear across the layer and Ū is the average speed of the two streams. Above a critical velocity ratio RCT = 1.32 the shear layer displays energetic oscillations at a discrete frequency which are the result of very organized axisymmetric vortex structures in the mixing layer. The spatialorder of the primary vortices inhibits the pairing process and dramatically alters the spatial developmentof the shear layer downstream. Consequently, the turbulence level inthe jet core is significantly reduced, as is the decay rate of the mean velocity on thejet centreline. The response of the shear layer to controlled external forcing indicates that the shear layer oscillations at supercritical velocity ratios are self-excited. The experimentally determined critical velocity ratio of 1.32, established for very thin axisymmetric shear layers, compares favourably with the theoretically predicted valueof 1.315 for thetransition from convective to absolute instability in plane mixing layers (Huerre & Monkewitz 1985).