Three isoforms of β-N-acetylhexosaminidase (β-NAHA), named β-NAHAs Ⅰ, Ⅱ and Ⅲ, were isolated from six-day-old etiolated mungbean (Vigna radiata) seedlings. β-NAHA Ⅰ was purified to apparent homogeneity by a procedure involving Con A-Sepharose chromatography, chromatofocusing, and gel filtration. β-NAHAs Ⅱ and Ⅲ were highly purified. β-NAHAs Ⅰ, Ⅱ and Ⅲ had molecular masses of 135, 127 and 110kDa, respectively. β-NAHA Ⅰ was dissociated into a single 67kDa protein band. Ⅱ was dissociated into two protein bands corresponding to 60 and 48kDa, and Ⅲ was dissociated into a single 48kDa protein band in SDS-polyacrylamide gel electrophoresis. The results suggest that isoforms Ⅰ and Ⅲ are homodimeric enzymes, each comprising two identical subunits with molecular masses of 67kDa and 48kDa, respectively, while isoform Ⅱ is a heterodimeric enzyme, comprising two non-identical subunits with molecular masses of 60kDa and 48kDa. All the enzymes were active against paranitrophenyl-β-N-acetylglucosaminide (PNP-β-N-acetylglucosaminide) and PNP-β-N-galactosaminide. The enzymes were inhibited by 5,5'-dithiobis (2-nitrobenzoic acid)(DTNB), Ag(superscript +), Hg(superscript 2+), and N,N'-diacetylchitobiose. Km values for isoforms Ⅰ, Ⅱ and Ⅲ were 0.67mM, 1.04mM and 1.76mM, respectively, using PNP-β-N-acetylglucosaminide as a substrate. These three isoforms had acidic p1 values (Ⅰ, 6.3; Ⅱ, 6.1; and Ⅲ, 5.9). Their optimal pH in the reaction towards PNP-β-N-acetylglucosaminide was 5.4, 4.7 and 5.7, and optimal temperatures were 65℃, 65℃ and 50℃ for isoforms Ⅰ, Ⅱ and Ⅲ, respectively.