In this paper, we introduce a formal-language model for explicitly formalizing agent-environment interaction in a multi-agent systems (MAS) framework: <i>Conversational Grammar Systems</i>(CGS). The main goal of the model is to provide a formal framework for defining how agents interact with environments in MAS. CGS offer a model with a high degree of flexibility, what means that they are able to accept new concepts and modify rules, protocols and settings during the computation. Evolution and action are involved in a consistent way in environment, while interaction of agents with the medium is constant. Based on a consolidated and active branch in the field of formal language theory, CGS are a highly formalized framework based in the postulates of artificial life that seems to be quite easy to implement, due to the simplicity of the formalism and the computational background of the theory they use.