Technical Abstract: Psoroptes ovis (Hering), commonly known as the sheep scab mite, is responsible for psoroptic scabies of cattle and sheep. A major P. ovis allergen, previously identified as a 16 kDa protein, was chosen as a candidate immunogen, purified and the sequence of the first 30 N-terminal amino acids was obtained (Pruett 1999). Reverse translation of the N-terminal amino acid sequence provided a degenerate nucleotide sequence, which was used to design oligodeoxynucleotide PCR primers. Use of the PCR primers with a P. ovis cDNA library succeeded in amplification of a 90 bp cDNA gene fragment which was cloned, sequenced and used to select unique sequencing/PCR primers. Primer walking generated overlapping subclones which yielded the 588 nucleotide consensus sequence of the cDNA encoding the 143 amino acid P. ovis allergen precursor. Nucleotide and translated sequences of the cDNA were compared with sequences in GenBank and found to be homologous to mite group 2 allergens Lep d II (formerly Lep d I) of Lepidoglyphus destructor, Der f II of Dermatophagoides farinae, Der p II of Dermatophagoides pteronyssinus, Tyr p II of Tyrophagus putrescentiae, Eur m 2 of Euroglyphus maynei and Gly d II of Glycophagus domesticus. The mature P. ovis allergen is composed of 126 amino acids with a calculated molecular mass of 13468 Da, 3 disulfide bonds, and pI of 6.06 with one potential o-glycosylation site at Thr116. We designate the P. ovis 16k protein as Pso o II to conform with nomenclature for mite group 2 allergens.