The breakup of an axisymmetric liquid jet, injected vertically upward from a nozzle into another immiscible liquid, into droplets is studied numerically. The unsteady motion of the interface separating two immiscible fluids is followed by solving the Navier-Stokes equations for incompressible and Newtonian fluids in axisymmetric cylindrical coordinates with a Front-Tracking method. The evolution of the interface and the specific surface area of the droplets are in good agreement with experimental results. Three breakup modes, dripping, jetting with uniform droplets, and jetting with non-uniform droplets, are identified. The different modes are shown on a Weber number-the viscosity ratio map. (c) 2006 Elsevier Ltd. All rights reserved.