A simple and sensitive nonchromatographic approach for indirect determination of sulfide at ultratrace levels in natural waters based on its selective precipitation with Hg2+ on the inner wall of a knotted reactor (KR) was developed for flow injection on-line sorption coupled with hydride generation atomic fluorescence spectrometry (HG-AFS). With the Hg2+ pH kept at 2.0, the HgS precipitation was formed in the KR after a reaction time of 120 s. A 10% (v/v) HCl was introduced to elute the remnant inorganic mercury and to merge with the KBH4 solution (0.05% m/v) for HG-AFS detection. Under the optimal experimental conditions, the sample throughputs were 20 h(-1). The detection limit was found to be 0.05 microg L(-1), and the relative standard deviation (RSD, n = 11) for determination of 2.0 microg L(-1) sulfide was 3.3%. The developed method was successfully applied to the determination of sulfide in a variety of natural water samples and wastewater samples with the gas-phase separation and sorption apparatus.