Catalyzes the hydroxylation of L-phenylalanine to L-tyrosine (PubMed:18460651, PubMed:10928216). Catalyzes the hydroxylation of tryptophan to 5-hydroxy-L-tryptophan (PubMed:10928216). Plays a role in the biosynthesis of a melanin-like cuticle pigment (PubMed:18460651). (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + L-phenylalanine + O2 = (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin + L-tyrosine (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + L-tryptophan + O2 = (4aS,6R)-4a-hydroxy-L-erythro-5,6,7,8-tetrahydrobiopterin + 5-hydroxy-L-tryptophan Inhibited by tetrahydrobiopterin. Unlike its mammalian orthologs, pah-1 does not exhibit allosteric binding behavior for phenylalanine. Amino-acid degradation; L-phenylalanine degradation; acetoacetate and fumarate from L-phenylalanine: step 1/6. Homotetramer. Expressed in the seam cells of the lateral hypodermis, in the ventral hypodermis and in the hyp7 hypodermal syncytium, in hypodermal cells in the tail and in body wall muscle cells (at protein level). Expressed during all larval stages and in adult animals (at protein level). Reduced L-phenylalanine hydroxylation. Lack of a yellow-orange pheomelanine-like pigment in the cuticle. Higher cuticle resistance to physical or chemical disintegration factors or oxidizing environments. Increase in superoxide dismutase activity. Increased life span. In a bli-3(e767) mutant background, growth arrest in early larval development, severe cuticle abnormalities with large blisters and increased superoxide dismutase activity. RNAi-mediated knockdown together with fah-1 RNAi partially rescues the impaired growth and fertility defects in the single fah-1 RNAi mutant (PubMed:18227072). Belongs to the biopterin-dependent aromatic amino acid hydroxylase family.