SUMMARY A new genus and species of Myxomycetes (order Physarales) is described. The organism, which has been maintained in laboratory culture for seven years, appears to form a link between the Echinosteliales and the Physarales, with closer affinities to the latter. Its known distribution is confined to the type locality (Boulder, Colo., U. S. A.) and it is known only from moist chamber culture of the bark of Ulmus americana. In general appearance it resembles Lamproderma, Comatricha, and Macbrideola but the subhypothallic development of its sporophore and its phaneroplasmodial assimilative stage preclude its inclusion in the Stemonitales. In May, 1965, one of us (CJA), while visiting in Boulder, Colorado, collected several pieces of bark from a living American elm tree (Ulmus americana L.) located just at the entrance of the Harvest House Motel. When placed in moist chambers (Gilbert and Martin, 1933) all these pieces of bark developed sporangia of a myxomycete which could not be placed in any known genus. Over a period of 7 yr the same organism has developed in moist chamber culture on six additional collections of bark-each consisting of several pieces-from the same tree and from two nearby elm trees, kindly sent by request to one or the other of us by various persons. Although through the years we have made bark cultures from hundreds of trees from many parts of the world, this unique species has not appeared on any substratum other than the bark from those three elm trees in Boulder. Attempts to put the organism in agar culture were immediately successful, and the life cycle was completed from spore to spore in 5 da.