In the semi-arid Mwea Division of Kenya's Eastern Province, the modern pigeonpea (Cajanus cajan) variety Nairobi Pigeonpea 670 (NPP 670) had become known to all farmers and was being grown by 68% of them within a period of 12 years. The only injection of this seed to the area was from a single on-farm demonstration. Three-quarters of farmers found out about the variety from observing it growing in the field, and obtained seed primarily from other farmers in the village. Factors favouring the diffusion of the variety included its attractiveness as a cash crop, the ease with which it could be distinguished from other varieties, the low seed rate, and the relative ease with which growers were able to maintain seed purity. Farmers expressed a willingness to pay for fresh seed, which suggests that more effort needs to be made to involve the formal seed sector. The present regulatory system does not favour the development of a formal supply system. NPP 670 was released more than 10 years after it was first tested.