The basidiomycetous red pigmented yeast P. rhodozyma is one of the most important biotechnological sources of the carotenoid pigment astaxanthin. This carotenoid has become very important in aquaculture of salmonids and crustaceans. Very little is known of the genetics of P. rhodozyma, and the ploidy of the yeast remains unknown. Recently, Golubev described the perfect state of P. rhodozyma, in which he found a pedogamic sexual process of conjugation in this yeast. We have studied the ploidy of several strains of P. rhodozyma by flow cytometry. A Beckton Dickinson FACScan flow cytometer was used to measure propidium iodide-stained DNA from the yeast cells. As an internal control we used two strains of Saccharomyces cerevisiae of known ploidy. All of the wild-type and mutant strains studied showed a ploidy level higher than haploid; most of them are polyploids and two of them are diploids. This research expands on the findings of other authors who compared the total DNA in the yeast cells by spectrophotometry and by pulsed field gel electrophoresis. Aneuploidy and/or polyploidy appears to be a common property of P. rhodozyma. The possession of multiple chromosomes might provide strains with a degree of protection from the effects of lethal mutations (and may explain the difficulty of obtaining auxotrophic mutants in P. rhodozyma), as well as giving rise to more vigorous strains in a manner similar to that in other industrial yeasts.