A method for investigating the breakup zone of the spray produced by a high-speed liquid jet is described. This uses measurements of the resistance between the nozzle and a wire, which is stretched across different positions in the spray, to derive the total cross-sectional area of unatomized liquid zones as a function of distance downstream. The new conductivity probe technique offers an improvement over previous methods by (1) using the correct theoretical treatment and a new data analysis method to relate signals to breakup zone dimensions, (2) providing an improved spatial resolution without the problems due to surface films on the bodies of ''needle'' probes, and (3) operating satisfactorily in the pulsed sprays produced by a commerical diesel injector and jump combination. The method is used with the pulsed spray from a 0.41-mm-diameter single-hole injector at atmospheric conditions, and the time variation of the breakup zone dimensions is derived. The breakup zone is found to reach a nearly steady length of 120 nozzle diameters during the main part of the spray pulse.