We report experimental results on the evolution of a laminar liquid jet injected with negatively buoyant condition in a miscible surrounding liquid. Since molecular di®usion is negligible, the only signi¯cant miscible e®ect is the absence of any surface tension. After an initial intrusion phase, the jet reaches a steady-state characterized by a constant penetration depth. A simple theoretical model is derived and successfully predicts the transient phase as well as the subsequent steady-state in terms of stationary penetration depth and jet's pro¯le. All the experimental points collapse on a master curve involving two dimensionless numbers: the densimetric Froude number Fr and S, a number comparing viscous friction to buoyancy. Finally, this curve obtained for laminar °ows is compared to classical results on turbulent fountains.