We present a formal theory of a behavioral L-system and describe as application a real time structured L-system interpreter. The behavioral L-system is a timed, conditional, stochastic, parametric and environmentally sensitive L-system which allows besides the modeling of plant and fractal development the description and animation of autonomous actors completely defined by production rules. A virtual environment including geometric objects, force fields and sounds and the behaviors of actors can be defined using the same formalism of the behavioral L-systems