Citrullinemia is an inborn error of metabolism due to deficiency of the urea cycle enzyme, argininosuccinate synthetase [L-citrulline:L-aspartate ligase (AMP-forming), EC 6.3.4.5]. The disease was first described in humans but was recently reported in dairy cattle in Australia. Here we report the nucleotide sequence of the normal bovine cDNA for argininosuccinate synthetase and the mutation present in animals with citrullinemia. Analysis of DNA from affected animals by Southern blotting did not readily identify the mutation in the bovine gene. RNA (Northern) blotting revealed a major reduction in the steady-state amount of mRNA in the liver of affected animals to less than 5% of controls. The bovine cDNA was cloned and sequenced and revealed 96% identity with the deduced human sequence at the amino acid level. Starting with mutant bovine liver, the mRNA was reverse-transcribed; the cDNA product was amplified with the polymerase chain reaction, cloned, and sequenced. The sequence revealed a C----T transition converting arginine-86 (CGA) to a nonsense codon (TGA). A second C----T transition represented a polymorphism in proline-175 (CCC----CCT). The mutation and the polymorphism were confirmed by amplification of genomic DNA and demonstration with restriction endonuclease enzymes of both the loss of an Ava II site in DNA from mutant animals at codon 86 and the presence or absence of a Dde I site at codon 175. The loss of the Ava II site can be used for rapid, economical, nonradioactive detection of heterozygotes for bovine citrullinemia.