The strong fluorescence poly-4-vinylaniline nanoparticles (PVN) has been prepared under ultrasonic radiation. Based on the fluorescence quenching of PVN by Cr(VI), a method for the selective determination of Cr(VI), without separation of Cr(III) in water, was developed. The reaction conditions between Cr(VI) and PVN were investigated in detail. The assay is characterized by short reaction time (<1 min even at 0 degrees C temperature), very few interference stable fluorescence signals (at least 2.5 h), simple instrument (common spectrofluorometer) and simplicity (a one-step assay). Under optimal experimental conditions, a limit of detection of 0.02 microg ml(-1) was achieved. The calibration curve was linear over the concentration range 0.1-13.0 microg ml(-1) with a correlation coefficient of 0.9984. The proposed method has been applied to the selective quantification of Cr(VI) in synthetic samples and waste-water samples with the satisfactory results.