Monascus purpureus NTU 601 is a strain that produces monacolin K, gamma-aminobutyric acid (GABA), and citrinin under solid culture conditions. Because citrinin is a mycotoxin and possesses nephrotoxic and hepatoxic effects, it has a negative impact on the acceptance of red mold rice by people. In this research, a simple and quick selection method for mutant strains with low citrinin production was designed based on the fact that citrinin possesses antibacterial activity for Bacillus subtilis and will form an inhibition zone around the colony of the Monascus strain. The mutant strain M. purpureus N 301 only produced 0.23 +/- 0.01 ppm citrinin, which was 50% less than that of the parent strain, and the monacolin K production was 481.29 +/- 7.98 ppm and maintained 91% productivity. M. purpureus N 310, the other mutant strain, produced 0.27 +/- 0.01 ppm citrinin, which was 41% less than that of the parent strain, and the monacolin K production was 526.29 +/- 5.54 ppm, which showed no significant changes when compared with the parent strain. The GABA content of the two strains was 5000 ppm, which is similar to that of the parent strain. The results showed that the method could be used to select red mold rice with low citrinin production.