Eukaryotic phosphomannomutases (PMMs) catalyze the interconversion of mannose-6-phosphate to mannose-1-phosphate and are essential to the biosynthesis of GDP-mannose. As such, plant PMMs are involved in ascorbic acid (AsA) biosynthesis and N-glycosylation. We report on the conditional phenotype of the temperature-sensitive Arabidopsis thaliana pmm-12 mutant. Mutant seedlings were phenotypically similar to wild-type seedlings when grown at 16 to 18°C, but died within several days after transfer to 28°C. This phenotype was observed throughout both vegetative and reproductive development. Protein extracts derived from pmm-12 mutant plants contained lower PMM protein and enzyme activity levels. In vitro biochemical analysis of recombinant proteins showed that mutant PMM protein was compromised in its catalytic efficiency (K_cat/K_m). Despite significantly decreased AsA levels in pmm-12 plants, AsA deficiency could not account for the observed cell death phenotype. Since, at restrictive temperature, total glycoprotein patterns were altered and glycosylation of protein disulfide isomerase was perturbed, we propose that a deficiency in protein glycosylation is responsible for the observed cell death phenotype.