An experimental study was conducted to investigate the effects of axial forcing on the flow structures near the nozzle exit in coaxial isothermal jets and jet diffusion flames. The jet was excited by adding a periodic velocity fluctuation ranging from 0 to 400% of the mean jet velocity at the tube resonating frequency. The phase-averaged axial velocity fluctuation at the jet centre was measured with a one-component LDV and phase-locked visualization using a light chopper and a phase-conditioning circuit was performed. The changes of large-scale structures in the near field of the jet are described from the visualization of horizontal and vertical cross-cut Mie scattering images. The flow structures of the forced isothermal jet are classified into three regions on the basis of the emergence of azimuthal structures and the periodic behaviour of vortex structures. The jittering of azimuthal structures was characterized by a forcing amplitude ratio and the velocity difference between the jet and the co-flowing fluid. In case of the forced reacting jet, flame heights were measured from video tape recordings of the sooting images of the flame. The dependence of flame height on the forcing amplitude ratio shows the existence of a flame-length elongation and reduction region. The flame elongation is found to be related to the suppression of the flame flickering by forcing. From the Mie scattering images and flame-length measurements, it is suggested that the intense mixing observed in the fully forced laminar jet and the reduction of the flame length is closely related to the development of azimuthal structures. (C) 1998 John Wiley & Sons, Ltd.