We observed formation of female flowers and seedpods, subsequent to the production of male flowers and pollen, on branch cuttings from a male Populus deltoides var. wislizenii tree. Labile sex expression is rare in the genus Populus, and this is the only recorded instance for this variety of P. deltoides. Mature trees exhibited the same phenomenon in the field and some released copious amounts of seed during 1997. These observations may explain apparent sexual lability within four populations of P. deltoides along the Rio Grande. If protandrous hermaphrodites exist in this species, observations of sexual lability in these field populations could be explained by year to year variation in the timing of census observations; i.e., earlier season observations would score these individuals as males. We conclude that P. deltoides var. wislizenii is a subdioecious species with occasional bisexual individual plants.