Highly sensitive and selective fluorimetric methods have been developed for sequential determination of ultratrace amounts of toxic metals Cr, Se, Mn present in mixtures and in biological samples. These are based on a novel oxidative action of Cr(VI), Se(IV) and Mn(VII) on nonfluorescent 2-(alpha-pyridyl) thioquinaldinamide, PTQA, in different acidic media, i.e., 0.9-0.54 M H2SO4 (for Cr), 0.05-0.15 M H2SO4 (for Se) and 0.033-0.833 M H3PO4 (for Mn) to produce intensely fluorescent oxidized product of the reagent, having excitation and emission wavelength maximum at 360 and 500 nm respectively. Sequential determination of Cr, Se and Mn in their mixtures at different mass ratios have been achieved with high accuracies. The method has been applied to the environmental analaysis of biological materials, e.g., aquatic fish samples after destruction of organic matter with HNO3-HClO4-H2SO4. The standard deviation for analysis is +/-0.003 for Cr; +/-0.004 for Se and +/-0.0014 for Mn in mixture while their corresponding values are +/-0.3, +/-0.3 and +/-0.5 in fish samples. The methods are simple, rapid and highly reproducible and do not require any cleanup or enrichment step.