Abstract: In Streptomyces antibioticus, there are two genes TyrA and ORF438 required for the melanin biogenesis. To investigate whether expression of these two genes in cotton can change cotton fiber colour, we modified the TyrA and ORF438 genes to make their codon usage closer to the codon preference of cotton fiber genes. The resulting versions of these two genes were referred to as dtyrA and dORF438, respectively. Vacuolar targeting signals were also added to their ends. Under the cotton fiber specific Ltp3 promoter, dORF438 and dtyrA were first transformed into model plant tobacco (Nicotiana tabacum). Molecular analyses showed that both the dORF438 and dtyrA genes were successfully expressed in transgenic plants, and the melanin deposition was observed in the trichomes of transgenic tobacco. Excitedly, when the same dORF438 and dtyrA expression cassettes were transformed into cotton (Gossypium hirsutum L.) by pollen tube pathway, the colour of cotton fiber changed from white to brown. Molecular analyses confirmed that both genes were transformed into cotton and expressed successfully. All these results indicate that the synthesized dOFR438 and dtyrA genes can work well in tobacco and cotton. Our study may provide a potential method for modifying the colour of cotton fiber.