Disproportionating enzyme (d-enzyme) is a plastidial α-1,4-glucanotransferase but its role in starch metabolism is unclear. Using a reverse genetics approach we have isolated a mutant of Arabidopsis thaliana in which the gene encoding this enzyme (DPE1) is disrupted by a T-DNA insertion. While d-enzyme activity is eliminated in the homozygous dpe1–1 mutant, changes in activities of other enzymes of starch metabolism are relatively small. During the diurnal cycle, the amount of leaf starch is higher in dpe1–1 than in wild type and the amylose to amylopectin ratio is increased, but amylopectin structure is unaltered. The amounts of starch synthesised and degraded are lower in dpe1–1 than in wild type. However, the lower amount of starch synthesised and the higher proportion of amylose are both eliminated when plants are completely de-starched by a period of prolonged darkness prior to the light period. During starch degradation, a large accumulation of malto-oligosaccharides occurs in dpe1–1 but not in wild type. These data show that d-enzyme is required for malto-oligosaccharide metabolism during starch degradation. The slower rate of starch degradation in dpe1–1 suggests that malto-oligosaccharides affect an enzyme that attacks the starch granule, or that d-enzyme itself can act directly on starch. The effects on starch synthesis and composition in dpe1–1 under normal diurnal conditions are probably a consequence of metabolism at the start of the light period, of the high levels of malto-oligosaccharides generated during the dark period. We conclude that the primary function of d-enzyme is in starch degradation.