Dimorphism in the chytridiomycete Blastocladiella emersonii is controlled by environmental factors. After 24 hours under CO2-poor conditions the fungus forms thin-walled, colourless zoosporangia. On substrate enriched with bicarbonate thick-walled red-brown sporangia are formed with a life-time of months. The film contrasts these 2 forms of development with microscopy and animations.