Within the last few years, heterologous gene expression in the chloroplast of the green algae C. reinhardtii has become increasingly important. In this system codon usage adaption plays an important role since it can drastically affect the quantity & quality of the expressed protein. In this thesis, effects of codon usage on the expression of plastidically encoded proteins of C. reinhardtii have been investigated. By establishing an in vivo codon test system, effects of different codon clusters on the expression of the D1-subunit of the PSII could be analyzed. Furthermore, two variants of the aadA-gene and the GFP-gene could be expressed in the C. reinhardtii plastome. The experiments show that especially rarely used Arg-codons drastically affect gene expression and that only the adaption of Arg-codon usage can result in higher yields of the expressed products. Another part of the thesis was to analyze the role of the prokaryotic tmRNA-tagging system, which is also postulated for some plastomes. However, none of the experiments so far gave hints for such a system in the C. reinhardtii chloroplast.