We present the complete nucleotide sequence of a cDNA encoding rat cyclophilin. The 743-nucleotide sequence contains a 42-nucleotide 5' noncoding region, a 492 nucleotide open reading frame corresponding to a translation product of 164 amino acids with a molecular weight of 17,874, and a 3' noncoding region of 209 nucleotides. Primer extension studies reveal the presence of one minor and two major transcription start sites. Southern blot analyses are consistent with as many as 20 copies of the cyclophilin gene and possible pseudogenes. Cyclophilin mRNA is expressed in virtually all types of tissues of rat and monkey and appears to have been highly conserved during mammalian evolution.