A full-length cDNA encoding cytosolic phosphoglycerate mutase (PGAM; E.C. 184.108.40.206) from potato (Solanum tuberosum L.) was isolated. RNA blots indicated that the accumulation of PGAM transcript was similar in all organs studied. Transgenic potato plants with either constitutive, or tuber-specific, decreases in PGAM expression were created. Constitutive reduction of enzyme activity of down to 25% of the control resulted in increased 3-PGA contents of up to 30% in leaves, but also led to reduced growth rates, probably due to a reduction of photosynthetic activity. Soluble carbohydrates and starch in leaves were reduced, and the tuber yield of the plants was lowered. Plants with 30% of the wild-type PGAM activity in leaves and 55% in tubers showed reduced amounts of starch in tubers also. Tuber-specific reduction of PGAM expression in transgenic potato plants had no effect on starch metabolism and tuber yield, although the 3-PGA level was increased. This indicates that 3-PGA concentrations in potato tubers are not important for regulating ADP-glucose pyrophosphorylase activity in the production of starch under some environmental conditions.