A series of mutants resistant to carotenoid biosynthesis inhibitors (norflurazon, fluridone, and nicotine) were obtained by a visual color detection method on medium plates containing the inhibitors in order to breed higher astaxanthin producers in a green alga Haematococcus pluvialis. Hybrid strains generated by protoplast fusion between these resistant mutants were found to possess two-fold increased levels of ploidy, and three-fold higher carotenoid formation than the parental and wild type strains.