Phenylketonuria (PKU) is a metabolic and genetic disorder caused by a phenylalanine hydroxylase (PAH) gene deficiency that raises Phe levels in organs. Dietary therapy involves an elimination diet and Phe-free items, which may alter microbiota. The study examined the oral and intestinal microbiomes of a 63-year-old PKU patient and a control man, living in rural areas. iSeq100 (Illumina) sequenced the stool and oral 16S rRNA gene V3-V4 region. PKU guts had more Firmicutes and fewer Bacteroidetes than control. Clostridia predominated in PKU, while Bacteroidia dominated in control. Oral Bacteroidetes. Firmicutes, Proteobacteria, and Fusobacteria phyla were similar in both men. The microbiome may differ from those fed a Phe-free diet from birth due to late diagnosis and treatment of PKU. Due to the age of the 63-year-old patient’s and late therapy, the results differ from earlier studies. No study has compared an older PKU patient’s gut and oral microbiomes.