The i locus of the medaka fish contains the tyrosinase gene whose product is the key enzyme required for melanin biosynthesis. The ib allele at this locus, also denoted as i 5, causes oculocutaneous albinism in homozygous carriers. Its albino phenotype is very weak, characterized mainly by small and varying sized melanophores in juveniles. Cloning and sequencing analyses of the tyrosinase gene for the i b allele revealed the presence of a 4.7-kb extra DNA fragment in the 5′ untranslated region, this being Tol2, a DNA-based transposable element of the hobo Activator Tam3 (hAT) family which had previously been identified as a cause of another mutant allele i4. Its insertion point was 85 bp upstream of the main transcription initiation site and 50 bp downstream of the CATGTG motif that has been suggested to be essential for the promoter function of the tyrosinase gene. The transcription level of the tyrosinase gene was decreased in ib/ib fish, compared with wild-type fish. The insertion is thus a likely cause of the weak albino phenotype. The Tol2 element transposes in a cut-and-paste fashion, and its excision is mostly imprecise, leaving some nucleotides and/or removing excess nucleotides. The i b mutant strain can thus be expected to serve as a source from which various other mutations in the promoter region can be derived.