1. The breeding of nishiki-goi (colorful carp) in Yamakoshi District, which is said to have begun in the 18 century, has now developed into an established business, because of the success in raising new breeds on one side, and of the increasing demand for carp on the other. This development was accelerated to an extreme degree, and ended up to have established the district as the greatest carp-producing center in Japan during the period of high Japanese economic growth. 2. Ever since the very beginning up to the present time, most nishiki-goi farms in this district have been on a small scale and carried on by farmers as their side job, depending entirely on the work of their family members. 3. In early days, nishiki-goi breeding was a very speculative business because the success in it depended on the mere luck of appearance of good mutations. But popularization of nishiki-goi appreciation among common people has brought great changes to it, that is, the increase in the amount of carp production and naturally following stability in the breeding business. 4. In order to answer the increasing demand for carp, farmers have managed to secure enough number of culture ponds by; 1 using irrigation ponds, 2 using watered rice fields, and 3 remaking rice fields into culture ponds. 5. Sales system, too, has been markedly improved and modernized, that is, main part of selling has gradually been changing from “spot sales” to “sales through brokers (koi-shi), followed by “sales through commission agents” and now by “sales by auction”. 6. Nevertheless, carp business is never free from ups and downs since general demand for carp wavers, as is often the case with so called “leasure industry”, in accordance with fluctuations of social-economic condition of the time. 7. The main reasons why this district has developed into the greatest producing center are considered to be; 1 enough number of irrigation ponds and amount of good water for carp, 2 years' ceaseless efforts to breed newer and better carp, 3 increasing demand created by the high economic growth and the government policy to decrease rice production, 4 the use of oxygen to transport carp alive and of vinyl pipes to supply water. 8. In this district the income from nishiki-goi now exceeds that from rice produce. This profit has meant economically a great deal to the people living there. It is owing to carp-business that this district, which used to be an isolated mountain village, has now been making great changes.