The separated shear layer in the near wake of a circular cylinder was investigated using a single hot wire probe, with special attention given to the shear layer instability characteristics. Without end plates to force parallel vortex shedding, the critical Reynolds number for the onset of the instability was 740. The present data, together with all previously published data, show that the ratio of the instability frequency f(sl) to the vortex shedding frequency f(v) varies as Re-0.65, which is in agreement with the Re-0.67 dependence obtained by Prasad and Williamson [ 1997, J Fluid Mech 333: 375 - 402]. However, the distribution of f(sl)/f(v) and the spectra of the longitudinal velocity fluctuation (u) suggest that, on either side of Re=5,000, the shear layer exhibits lower and upper subcritical regimes, in support of the observations by Norberg [ 1987, publication no. 87/2, Chalmers University of Technology, Sweden] and Prasad and Williamson [ 1997, J Fluid Mech 343: 235 - 265]. The spectra of u provide strong evidence for the occurrence of vortex pairing in wake shear layers, suggesting that the near wake develops in a similar manner to a mixing layer.