Kozo is a kind of mulberry species, botanical point of view it is hybrid of “Broussonetia kazinoki” and “Broussonetia papyrifera”. It is the most popular material, roughly more than 90% of Washi since 1000 years ago.

Generally, there are six species as below, classified in two groups. Kozo's fibre is shorter than Kaji's.
  => Kozo : Aka(Red)-so, Ao(Green)-so
  => Kaji : Taori, Kuro(Black)-kaji, Taka(High)-kaji, Ma-kaji
Even so, it is not strictly separately using. The place of the farm is more importantly recognized for its quality and characteristic. In Japan, north latitude 40 degrees is the northern limit, in Iwate and Akita prefecture.
Some of the famous farm-land is as below.
  => Tosa (Kochi pref.): The biggest production place in Japan.
  => Nasu/Daigo (Ibaraki pref.): Recognized as the highest quality. The fibre is very short and thin.
  => Higo (Kumamoto/Fukuoka pref.): The longest fibre Kozo, occasionally more than 4cm. In the group of Kaji.
And also, each Washi making districts cultivate by themselves. It makes each paper have uniqueness.
Although it is very rare, some artisans use wild species. It could be more closer to the original ancient Washi.
The imported material is also widely used as cost-conscious, from China, Philippines, Thailand and Paraguay.