A species of Zygogonium forms extensive dark purple mats in Yellowstone National Park in acidic habitats adjacent to thermal areas. These mats range up to 6 cm in thickness and up to 3000 m2 in areal extent. Temperatures in the mats varied from 20–31 C and pH varied from 2.4–3.1. These mats form on soil in areas where a moist surface is created by the presence of small acidic springs or seeps. The effect of light, temperature, and pH on photosynthesis was studied in the field by use of 14CO2. Photosynthesis increased in rate up to full sunlight; light inhibition was not observed. Temperature optimum for photo-synthesis was 25 C. A broad pH optimum was found between 1.0 and 5.0. The Zygogonium mats have a high water holding capacity and create a moist habitat in which Euglena and Chlamydomonas develop. The mats also serve as repositories for the eggs of the brine fly Ephydra bruesi and both larvae and adults of this fly probably consume Zygogonium filaments as their main food source.