(1) Fronds of Rumohra adiantiformis-a creeping evergreen terrestrial fern of pantropical and southern temperate distribution which is common in the indigenous forests of the southern Cape, South Africa-were harvested commercially in state forests for the first time in 1982 and exported for use as florists' greenery. (2) Reduced availability of fern fronds led to this study of the fern's population structure, phenology and growth and of the effects of harvesting on frond production. Marketable fronds were harvested at 8-25 week intervals from experimental plots set out in natural R. adiantiformis populations. Frond yields per unit area, and frond turnover on individual plants were recorded over a period of 34 months. The commercial frond harvest and its effects were monitored at seventeen other sites. (3) In disturbed forest most R. adiantiformis plants are small, but in high closed forest most are large. Small plants grow very slowly. Large plants produce 1-4 (mean 1 2) fronds annually and fronds live for up to 34 months. There are 1-7 fronds per rhizome, the number varying seasonally. Frond size is related to rhizome diameter, with only large fronds bearing sori. (4) A mean fresh weight of 46 7 kg ha' yearof marketable fronds was harvested in a virgin stand, but the sustainable yield of marketable fronds from natural populations in the Southern Cape is less than 16 kg ha-' year-'. In the first year of commercial exploitation, frond removal exceeded frond production, but offtake decreased with continued cropping. (5) Experimental and commercial harvesting of R. adiantiformis fronds at 8-25 week intervals led to a decrease in the standing crop of fronds and in the length, mass and fertility of new fronds produced. Harvested plants produced unseasonal frond buds, and in some cases, lateral rhizome branches. Recovery of defoliated plants was slow, no increase in frond size being apparent after 22 months.