There are several studies which support the contention that some tropical rain forests (trf) have more plant species on small areas of up to a few hectares than any other kind of vegetation on earth (Parsons & Cameron 1974, Whitmore 1984a). Next richest, but far poorer, are Mediterranean heaths in South Africa (fynbos) and Australia (Figure 1) which have about the same total number of species as there are tree species >10 cm diameter at breast height (dbh) in trf. In fact, statements about species richness in trf are based almost entirely on counts of tree species over a given minimum dbh, usually 10 cm. The range per hectare is from about 20 to a maximum count of 223 at Mulu, Sarawak (Proctor et al. 1983, Whitmore 1984b). Here we report a count of 233 vascular plant species on just 100 m2 of a lowland trf in Costa Rica. This is equivalent to about one-sixth of the total flora of the British Isles on half the area of a singles tennis court (97.8 m2). There were also 32 bryophytes, all epiphytic; non-vascular plant species have not been included in previous counts in trf. The count was made by destructive sampling of a 10 X 10 m plot in lowland trf on a broad ridge-top 2 km west of the Rio Puerto Viejo near Horquetas (840 05' W, 100 22' N), about 6 km SSE of Finca La Selva, in the Atlantic lowlands of Costa Rica, at an elevation of about 100 m. The forest reaches 50 m tall. Pentaclethra macroloba (Willd.) Kuntze is the commonest big tree (as in the well-known La Selva forests) but it is not the tallest. Mean annual rainfall at La Selva is about 4000 mm with a relatively dry season January to April (Frankie et al. 1974 give fuller details). The total numbers of species and individuals in different synusiae (life form classes) are shown in Table 1 (voucher specimens are held at Oxford). Most remarkable is the very large number of species, 132 or 57% of all vascular