BIOSIM -- A biological neural network simulator for research and teaching, featuring interactive graphical user interface and learning capabilities BIOSIM is a biologically oriented neural network simulator, which is available as public domain software. BIOSIM was designed for research and teaching. Hence, the ease of handling was of prime importance. This task was achieved by implementing a graphical user interface and presenting it as a turnkey system which avoids the necessity to learn complex programming languages.par In BIOSIM, four neuron models are implemented: a simple model only switching ion channels on and off, the original Hodgkin-Huxley model, the SWIM model (a modified HH model) and the Golowasch-Buchholz model as the most enhanced model. Dendrites consist of a chain of segments without bifurcation. Three synaptic types are predefined (excitatory, inhibitory and electrical). Additional synaptic types can be created interactively. Biologically oriented learning and forgetting processes are modeled, e.g. sensitization, habituation, conditioning, hebbian learning and competition learning. A neural network can be created by using the interactive network editor which is part of BIOSIM. Parameters can be changed via context sensitive menus and the results of the simulation can be visualized in orbservation windows for neurons and synapses. Stochastic processes such as noise can be included.par A demonstration shell script, which controls the simulator, is included showing a wide range of possibilities with BIOSIM. A hypertext based online-help system is embedded. BIOSIM was designed as a bilingual (English and German) system, which can easily be augmented by other languages.