This report describes the mutational spectrum and linked haplotypes of the phenylalanine hydroxylase gene in Venezuela. In this study, we have detected European mutations such as IVS10nt-11, R243Q, and R408W on the same haplotype background (6.7, 1.8, and 2.3, respectively) as in Europe. In this sample, we have found two novel mutations: S349L detected in two homozygous siblings on the background of haplotype 6.7, and a small deletion, P314fsdelC, that results in a frameshift and a premature stop codon detected on the background of haplotype 4.3. The definite demonstration that mutation S349L results in a nonfunctional protein was shown by expression analysis in prokaryotic and eukaryotic systems. This mutation results in an unstable phenylalanine hydroxylase (PAH) protein completely devoid of enzymatic activity well correlated with the severe form of the disease exhibited by the homozygous patients.