Electrostrymon denarius is the first mushroom-feeding butterfly caterpillar discovered in the New World. It belongs to the Calycopidina, a subtribe of lycaenid butterflies whose caterpillars eat detritus and seeds in the leaf litter. Electrostrymon denarius has not been reared previously, and we illustrate and briefly describe the biology and morphology of its caterpillar and pupa. The significance of this discovery is that it increases the range of organic leaf litter substrates that Calycopidina caterpillars will eat. Increased diet breadth may decrease the likelihood of species extinction.