Cylindrocarpon heteronema (Berk. & Br.) Wollenw. was obtained from perithecia of Nectria galligena Bres. which occurred on the bark of galled branches of Swietenia mahagoni Jacq. collected in Fort Lauderdale, Florida. Hyphal tips were taken from germ tubes of single cells from multiseptate macroconidia and from each cell of germinating ascospores. Nuclear staining revealed that the cells of the macroconidia and ascospores are uninucleate. All hyphal tips produced perithecia on carnation leaf pieces resting on water agar. This study establishes the homothallic nature of this fungus.