Citrinin, a hepato-nephrotoxic compound to humans, can be produced by the food fermentation microorganisms Monascus spp. In this study, we investigated the distribution of mycotoxin citrinin biosynthesis genes in 18 Monascus strains. The results show that the acyl-transferase and keto-synthase domains of the pksCT gene encoding citrinin polyketide synthase were found in Monascus purpureus, Monascus kaoliang, and Monascus sanguineus. Furthermore, the ctnA gene, a major activator for citrinin biosynthesis, was found in M. purpureus and M. kaoliang, but was absent in M. sanguineus. The orf3 gene encoding oxygenase, located between pksCT and ctnA, was also present in M. purpureus and M. kaoliang. The pksCT gene was highly conserved in M. purpureus, M. kaoliang, and M. sanguineus, while the ctnA and orf3 genes were shown to be highly homologous in M. purpureus and M. kaoliang. In contrast, the PCR and Southern blot analyses suggest that pksCT, ctnA, and orf3 were absent or significantly different in Monascus pilosus, Monascus ruber, Monascus barkeri, Monascus floridanus, Monascus lunisporas, and Monascus pallens. A citrinin-producing phenotype was detected only in M. purpureus and M. kaoliang using high performance liquid chromatography (HPLC). These results clearly indicate that the highly conserved citrinin gene cluster in M. purpureus and M. kaoliang carry out citrinin biosynthesis. In addition, according to the phylogenetic subgroups established with the beta-tubulin gene, the citrinin gene cluster can group the species of Monascus.