A dynamic economic model for a biodiversity prospecting contract, between a host country and a pharmaceutical company, is developed and used to explain the structure of existing contracts. The host country's stocks of biodiversity and genetic information are crucial inputs to the production of high-quality samples. Even with compete property rights contracts will be second best; it is not possible to perfectly monitor host-country inputs to the drug discovery process. Contracts vary due to the different degrees of observability of host-country inputs, and incomplete or ineffective property rights.