Achlya bisexualis is an oomycete in the family Saprolegniaceae that decomposes organic matter in freshwater habitats and can act as a pathogen of fish and invertebrates under certain conditions. It exhibits a fascinating pheromone-based sexual reproduction system that has been extensively studied as a model for steroid hormone signaling in lower eukaryotes. Its mycelial colonies rapidly colonize dead insect and plant debris in slow-moving water.