Abstract:The plant materials used were the leaves of Aegilops squarrosa L., Triticum aestivum L. and a nucleo-cytoplasmic hybrid wheat which was obtained by crossing T. aestivum ( ♂ )with A. squarrosa( ♀ )and backcrossing 10 generations with T. aestivum as the paternal parent. The carboxylasc and oxygenase activities of these materials were assayed. At the same time, a λgtl0 eDNA library of 5.5 x 105 pfu was constructed with poly (A)RNA from the leaves of A. squarrosa seedlings. This library was screened with a partial fragment of rice rbeS, and a eDNA clone pRAS1 that includes A. squarrosa rbcS was obtained. Nucleotide sequencing indicated that the full length of this eDNA was 815 bp. Con‘son of small subunit of ribulosc-l,5-bisphosphate carboxulase/oxygenase (rubisco) from A. squarrosa with that from T. aestivum and their nucleo-cytoplasmic hybrid wheat revealed that the amino acid residues at positions 56, 84, 98 and 117 in the small subunit might play important roles in regulating the activities of mbisco.