||ATP-Binding Cassette (ABC) proteins are transmembrane transporters that utilize the energy of ATP hydrolysis to translocate their substrate across biological membranes, they have been found in every eukaryotic genome so far. In human genome, there are 49 ABC genes divided into seven subfamilies. My study mainly focuses on the expression pattern of ABC homologues in Chlamydomonas reinhardtii. Using bioinformatics search, I identified 86 putative ABC genes in the genome of C. reinhardtii, and 37 of them have homologous sequence in C.reinhardtii ESTs. From these 37 genes, I selected one or two representative members of each subfamily to design primers and carried out RT-PCR. The RT-PCR products obtained from 6 pairs of primers showed predicted sequences and have been tentatively identified as the homologues of human ABCA5, ABCB1, ABCC4, ABCE1, ABCF1 and ABCF2 genes, respectively. Semi-quantitative RT-PCR using RNA extracted from cells at different growth stages has established the expression patterns of these 6 ABC genes in normal growth condition. The expression of ABCA5 homologue was the most consistent, while ABCB1 and ABCC4 homologues varied in wider ranges. Stress conditions such as darkness, cold treatment (1O°C), NaCl, sorbitol or CdCl2 addition, and N-deficiency exert different effects on the expression of each putative C. reinhardtii ABC gene. The expression of ABCC4 homologue varies in the widest range under different conditions and growth stages, and ABCE1 homologue has the highest degree of homology with human ABCE1 gene. Therefore, these two genes were selected as the study target in P-deficient stress and P-deficient with Cd added stress. The results showed that the effect of P-deficient with Cd added stress was higher than the effect of P-deficiency only and was different from the effect of Cd stress on ABC gene expression pattern.