The absence of L-ascorbic acid (L-AA, or AA) synthesis in scurvy-prone organisms, including humans, other primates, guinea pigs, and flying mammals, was traced to the lack of L-gulonolactone oxidase (GULO) activity. GULO is a microsomal enzyme that catalyzes the terminal step in the biosynthesis of L-AA. Clinical cases of scurvy were described in a family of Danish pigs. This trait is controlled by a single autosomal recessive allele designated od (osteogenic disorder). Here we demonstrate that the absence of GULO activity and the associated vitamin C deficiency in od/od pigs is due to the occurrence of a 4.2-kbp deletion in the GULO gene. This deletion includes 77 bp of exon VIII, 398 bp of intron 7 and 3.7 kbp of intron 8, which leads to a frame shift. The mutant protein is truncated to 356 amino acids, but only the first 236 amino acids are identical to the wild-type GULO protein. In addition, the od allele seems to be less expressed in deficient and heterozygous pigs compared with the normal allele in heterozygous and wild-type animals as determined by ribonuclease protection assay. We also developed a DNA-based test for the diagnosis of the deficient allele. However, we failed to identify the mutated allele in other pig populations.