The catalytic effect of manganese (II) on the oxidation of 3 - Methyl - 6 - (2 - hydroxyethoxy) - 2 - [2 - methoxy - 4 - N (N,N - diethylamino) phenylazo] benzothiazolium methylsulphate (MHMDPBM), with potassium periodate in the presence of 1,10-phenanthroline in weakly acidic media was studied. The reaction was followed spectrophotometrically by measuring the decrease in the absorbance of the dye at 500 nm. Under the optimum conditions (4x10(-5) mol dm(-3) MHMDPBM, 4x10(-4) mol dm(-3) potassium periodate, 1x10(-4) Mol dm(-3) 1,10-phenanthroline, 0.1 mol dm(-3) buffer - pH 3.0, 70degreesC, 8 min), manganese (11) in the range 0.05-5 ng cm(-3) can be determined by the fixed-time method with a detection limit of 0.015 ng cm(-3). The influence of foreign ions on the accuracy of the results was investigated. The developed method is extremely sensitive, selective, and simple. The method was applied successfully to the determination of total manganese in strawberries, raspberries and bilberries. The results showed good agreement with those obtained by atomic absorption spectrophotometry.