Monosporous cultures have been secured from 11 different collections from the Florida Keys of Schizophyllum umbrinum Berk., a small brown species restricted to the region of the Caribbean, In culture these mycelia produced normal fruiting bodies readily and with high frequency. No evidence of mycelial interaction, either dikaryosis or heterokaryosis, was found in intra- or interstock matings or in phase microscopic examination of nuclear content in mycelia derived from single spores, from matings, or from the stripes of fruiting bodies. The vegetative mycelia exhibited a strong tendency to undergo marked, permanent, and presumably mutative, degeneration.