Chrysomyxa weirii (Uredinales) is the only autoecious, microcyclic species of Chrysomyxa occur- ring in North America. The telia form on second-year needles of spruce, causing premature needle loss. The morphology of the telia was studied in herbari- um specimens from diverse locations, and the telio- spore germination, nuclear condition, and reproduc- tive biology of fresh collections were studied on mi- croscope slides and on artificially and naturally in- fected host tissue using light and scanning electron microscopy. Basidiospore production was infrequent in mature sori, but teliospores dispersed readily in water and germinated to produce a two-celled basid- ium and two basidiospores. The two cells of the ba- sidium could also separate to form two sporelike cells that could produce germ tubes, or the teliospore pro- duced a long hyphalike promycelium. The type of germination was influenced by temperature. The ready dispersal of teliospores in water and their pres- ence on the surface of current-year needles confirms that they function as diaspores. The distribution pat- tern of this rust and the elongated, smooth, thin- walled spores that are held rigidly together until wet suggest a water-dispersal mechanism. A cytological study showed that the vegetative hyphae are mostly monokaryotic. Dikaryotization and karyogamy occur in the cells at the base of the telium and result in teliospores with one large nucleus. During germina- tion, the teliospore nucleus migrates into the basidi- um, where it divides once before a septum forms. A