Recent field work and collections from the Atlantic lowlands of Costa Rica have revealed a hybrid between two exindusiate species of Cyathea. This hybrid is noteworthy because it combines the characters of two species that are rather dissimilar and morphologically distinctive. Both of the parents are acaulescent tree ferns of reduced laminar architecture; both are also rather rare and spottily distributed, known from relatively few collections, and one was described only four years ago (Stolze, 1984). The hybrid in question is a cross between Cyathea stolzei A. R. Smith ex Lellinger [originally described as Trichipteris pinnata Stolze], from Costa Rica and Panama, and C. ursina (Maxon) Lellinger [Trichipteris ursina (Maxon) R. Tryon], occuring from Belize to Panama. Both parents and the hybrid were found growing together at the same locality: Costa Rica, Pcia. Heredia, Cerros Sardinal, ca. 2 km N of Chilamate de Sarapiquf (Finca La Martita), 10? 28' N, 84? 04' W, ca. 80-160 m, Smith et a]. 1780 (CR, MO, UC). Cyathea stolzei [Smith et al. 1794 (CR, MICH, MO, NY, UC)] was a relatively common fern in the forest understory, while C. ursina [Smith et al. 1814 (AAU, CR, MICH, MO, NY, UC)] was common in the forest and along trails, especially in somewhat swampy sites. A single plant of the hybrid was discovered growing along a trail at the edge of the forest. The area was one rather rich in ferns, with about 70 species collected, but the only other tree fern species that were found in the area were Cyathea trichiata (Maxon) Domin [Trichipteris trichiata (Maxon) R. Tryon], a species characterized by its bipinnate-pinnatifid fronds, abundantly hairy axes, and trunks several meters tall, and C. multiflora J.E. Smith, also with bipinnate-pinnatifid fronds, a small hemitelioid indusium, and trunks 1-2 m tall. One additional collection of the hybrid has been seen from an adjacent property: Costa Rica, Pcia. Heredia, Finca El Bejuco, at southern end of Cerros Sardinal (N of Rio Sarapiqui), Chilamate de Sarapiquf, ca. 100 m, 10? 27' N, 84? 04' W, Grayum et al. 4613 (MO). With it also grew the putative parents, C. stolzei (Grayum et al. 4608, CR, MO, UC) and C. ursina (Grayum et al. 4621 CR, MO). One additional tree fern was found at this site: Cyathea microdonta (Desv.) Domin; it is easily distinguished by its bipinnate-pinnatifid fronds and spiny rachis and costae. At El Bejuco, C. stolzei grows only on the ridgetops and upper slopes; C. ursina occurs only in the swampy stream bottom at the base of the ridge. The hybrid is in the intermediate zone-the lower slopes of the ridge. Of the two parental species, only C. ursina has been found from the fern-rich (173