The ‘plug’ flow emerging from a long rotating tube into a large stationary reservoir has been used in an experimental investigation of centrifugally unstable swirling jets. A moderate Reynolds number, $\hbox{\it Re}\,{=}\,1000$, was studied extensively, and swirl numbers, $S$, the ratio of nozzle exit rotating speed to the mean mass axial velocity, were in the range 0–1.1. Four regimes were covered: non-swirling jets with $S\,{=}\,0$, weakly swirling jets in the range $0\,{<}\,S\,{<}\,S_{c1}$, strongly swirling jets before vortex breakdown with $S_{c1}\,{<}\,S\,{<}\,S_{c2}$, and stable vortex breakdown for $S\,{>}\,S_{c2}$, where $S_{c1}\,{=}\,0.6$ and $S_{c2}\,{=}\,0.88$.