We isolated cDNA corresponding to open reading frame (ORF) 16 of the 81 kb contig of Arabidopsis thaliana chromosome III [Quigley et al., Nucleic Acids Res., 24, 4313-4318 (1996)] and expressed alpha -mannosidase activity in tobacco suspension-cultured cells, which revealed that ORF16 encodes alpha -mannosidase. We also suggested that Arabidopsis harbors three genes encoding alpha -mannosidase by homology search against the database.