Although most eukaryotic cells are sensitive to the 80S ribosome inhibitor cycloheximide (CYH), naturally occurring CYH resistance is widespread amongst yeast species. The primary determinant of resistance appears to be a single residue within ribosomal protein L41; resistance is acquired by the substitution of a conserved proline (P-56) by a glutamate residue. We have isolated the L41 gene (RPL41) from the green alga Chlamydomonas and investigated the molecular basis of CYH resistance in various mutant strains. In both the wild-type strain and the mutant act-1, a proline is found at the key position in L41.; However, analysis of six independently isolated act-2 mutants reveals that all have point mutations that replace the proline with either leucine or serine. Of the two changes, the leucine mutation confers significantly higher levels of CYH resistance. This work identifies the ACT-2 locus as RPL41 and provides a possible dominant marker for nuclear transformation of C. reinhardtii.