Unsteady flow past a circular cylinder at a Reynolds number of 300 has been simulated. The spanwise extent of the computational domain has been systematically varied from zero (2-D) to a large value of 12&, where 4 is the spanwise wavelength of the fundamental mode-B 3-D instability (Williamson, 1996). Simulations with intermediate spanwise extent of 4 and U, have also been performed so as to isolate the role of mode-B instability, its spanwise subharmonic instability and other longer wavelength instabilities. The results suggest that while imprints of mode-B and its subhatmonic instability can be observed in the near wake region, further subharmonic instabilities of larger wavelength are feasible downstream, leading to a rearrangement of the vertical structure and its spectral content. Furthermore, the prediction of global quantities such as lift and drag improves as the spanwise domain size is increased.