Asplenium is one of the largest and most interesting genera of ferns in the neotropics. Many of its species are distinctive and quite easy to identify, but a few others, notably A. auritum Swartz, A. cuspidatum Lam., and A. radicans L., are so highly variable that intensive monographic studies will be required to define their specific and infraspecific limits. During my studies of the genus for the Ferns and Fern Allies of a total of 40 species (including a number of varieties) have been recognized in this small Central American country. One of these is new. Asplenium williamsii Stolze, sp. nov. Figs. 14. Rhizoma erecta, paleacea; paleae lanceolatae vel lineares, fuscae, clathratae, 4-6 mm longae, 0.5-0.8 mm latae, plerumque attenuatae; folium pinnatum, 15-45 cm longum, 4-6 cm latum, ad apicem pinnatifidum vel serratum gradatim decrescens; petiolus 4-9 cm longus, plumbeus vel fuscus, anguste vel late alatus; pinnae 14-20(22)-jugae, obtusae vel subacutae, serratae vel biserratae; venae acroscopicae plerumque 1-furcatae, venae basiscopicae simplices: sori lineares, 3-8 mm longi, 0.5-0.8 mm lati, 1-2 sori proximales diplazioides. TYPE: Terrestrial in cloud forest, Montaiia Canahui, Depto. El Progreso, Guatemala, alt. 1,600-2,300 m, Steyermark 43791 (US; isotype F). In wet forests, commonly on the forest floor, but rarely epiphytic, 1,250-2,300 m; Alta Verapaz; Baja Verapaz; El Progreso; El Quiche; San Marcos; Santa Rosa. Mexico (Chiapas). Plants terrestrial, rarely epiphytic; rhizome stout, erect, amply provided with lanceolate or linear, lustrous, grey-brown, clathrate scales, these 4-6 mm long, 0.5-0.8 mm broad, mostly attenuate; leaves pinnate, subcaespitose, mature ones 15-45 cm long, 4-6 cm broad; petiole stout, 4-9 cm long, much shorter than the lamina, dull grey or grey-brown, glabrous, abaxially terete, adaxially flattened and narrowly to broadly green-alate (especially toward the lamina), each of the wings 0.3-0.8 mm broad; lamina linear to narrow-elliptic, glabrous, thinto firmmembranaceous, slightly reduced at base, gradually reduced to a pinnatifid or serrate apex, not proliferous; rachis glabrous, dull grey or reddish brown, greenalate throughout; pinnae 14-20(22) pairs, the middle ones 2-3.5 cm long, 0.8-1.5 cm broad, sessile to short-stalked, spreading to slightly ascending, approximate to subdistant, oblong to lanceolate, obtuse to subacute, inequilateral at the base, basiscopically cuneate or excavate, acroscopically truncate and often auriculate or subauriculate, the margins obtusely or subacutely serrate to biserrate; veins on the acroscopic side commonly once-forked, the basal one twice-forked, the distal ones and those of the basiscopic side simple, distinct adaxially, indistinct abaxially, the tips not or scarcely enlarged, ending well short of the margin; sori relatively long, often nearly reaching from midrib to margin (but tending to be more inframedial), linear, straight to slightly curved, 3-8 mm long, 0.5-0.8 mm broad, 1-2 proximal ones commonly double (diplazioid); indusium delicate, linear, pale yellowish to light brown, or hyaline, subentire.