We are developing a new software methodology for building large, complicated systems out of simple units. The emphasis is on the architecture which is used to combine the units, rather than on the intelligence of individual units. We named the methodology "organic programming" after the flexibility of organic systems such as plants and animals. In this paper, we describe application of an organic programming language Gaea to multi-agent systems. One of the advantages resides in that we can program the system in a subsumptive manner.