Arabidopsis mutants affecting ADPG pyrophosphorylase (ADGase) activity can be divided into two complementation groups, adg1 and adg2. Previous biochemical studies of adg2-1 mutant indicated that mutant plants do not accumulate ADGase large subunit protein and that ADGase small subunits assemble as homotetramers. This suggested that the ADG2 gene may encode the large subunit of ADGase. In this paper, it is shown that adg2-1 mutant plants accumulate near wild-type levels of transcripts encoding both the large and small subunits of ADGase. However, by RFLP analysis and complementation of adg2-1 with the ADGase large subunit gene, we show that the adg2-1 mutant does represent a mutation of the ADGase large subunit gene. Sequence analysis of the adg2-1 allele revealed a missense mutation. The results therefore suggest either that the missense mutation affects the stability of the ADGase large subunit protein or that it prevents assembly of the large subunit into holoenzyme.