Electromagnetic nanonetworks form an exciting new field of investigation. The frequency band and the channel access methods impose peculiar constraints on upper layer protocols. Given that research is impaired by the unavailability of ready to use hardware, simulation tools are needed to evaluate protocols and applications. This paper presents BitSimulator, a network simulator specifically targeting wireless nanonetworks. It is designed to accurately simulate collisions up to the bit level, allowing studies on coding, channel access, routing or congestion control. Being dedicated to this specific environment, it is highly optimized for speed and supports a large number of nodes.