The horseshoe vortex system resulting from the interaction between a laminar boundary layer and a round transverse jet was studied over a range of Reynolds numbers and velocity ratios using hydrogen bubble wire visualization in a water channel. The study shows that the horseshoe vortex system can be steady, oscillating, or coalescing, depending on the flow conditions. Topological concepts are used to interpret the observed flow patterns and compare these patterns with those observed and computed upstream of wall‐mounted circular cylinders. The Strouhal numbers of the observed oscillating and coalescing systems agree reasonably well with those appearing in the literature for wall‐mounted circular cylinders. The relationship between the unsteady horseshoe vortex motions and the unsteady vortex motions in the wake is studied for a velocity ratio of 4. Here it is shown that the oscillating regime occurs at the same frequency as the wake and the coalescing regime occurs at approximately double the frequency of the wake. The results indicate that the wake intermittently becomes coupled to the horseshoe vortex motions and that this occurs either at the horseshoe vortex frequency in the case of the oscillating system or a subharmonic in the case of the coalescing system. ©1995 American Institute of Physics.