Chlamydomonas reinhardtii can utilize CO2 for autotrophic growth (HSM plus 5% CO2) or acetate for mixotrophic growth (TAP). This study was to elucidate the differential regulation of methionine sulfoxide reductase (MSR) gene expression between HSM plus 5% CO2 and TAP cultured cells, and also to determine the difference of gene expression in response to high light (1,000 £gE m-2 s-1). The role of photosynthetic electron transport (PET) in the regulation of MSR gene expression was also examined by the use of PET inhibitors. High light inhibited PSII activity (Fv/Fm and Fv'/Fm') of HSM plus 5% CO2 and TAP cultured cells., while the responses of CrMSR gene expression in mixotrophically grown cells were different from autotrophically grown cells, High light increased the expression of CrMSRA1, CrMSRA2, CrMSRA3, CrMSRA5, CrMSRB1.2, and CrMSRB2.1, but inhibited the expression of CrMSRA4 and CrMSRB2.2 in autotrophically grown cells. The expression of CrMSRA3, CrMSRA5, and CrMSRB2.1 in mixotrophically grown cells was increased by high light but that of CrMSRA1, CrMSRA4, and CrMSRB2.2 was inhbited. The number of MSR isoform that was up-regulated by high light was greater in autotrophically grown cell than that in mixotrophically grown cells. Using the PET inhibitors (3-(3,4-dichlorophenyl)-1,1- dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB)), most of the CrMSRA expression was regulated by reduced QA for autotrophically grown cells while reduced PQ was the main site for mixotrophically grown cells by high light. The expression of CrMSRB in autotrophically grown cells was mainy modulated by QA (-) or Cytb6f (-), while that was not affected by PET, except a role of Cytb6f (-) on the high light-induced CrMSRB2.2 expression. We fouind that CrMSRB gene expression in autotrophically grown cells was highly affected by PET but not for micotrophically grtown cells. The present result that H2O2 did not accumulate in autotrophically and mixotrophically grown cells suggests that H2O2 may be not involved in the regulation of high light regulation of CrMSR gene expression. The present study shows that the mRNA expression of CrMSR isoforms in Chlamydomonas was diffrerentially regulated between autotrophically and mixttrophically grown cells. The relationship between the utilization of different C source and CrMSR gene expression will be discussed.