Experimental observations of the breakup of a liquid jet issuing from a vibrated nozzle support the view that particles passing through the orifice tend to create a surface disturbance which, depending on the phase relative to the nozzle vibration, may either add to or subtract from the disturbance created by the nozzle vibration. The result may be either a shortening or lengthening of the jet breakoff length, depending on phase. The implications for sorting large cells with a fluorescence-activated cell sorter are discussed.