Interspecific hybrids have been set up between a diploid species of Cydosorus from Ceylon provisionally identified as C. repandulus and three tetraploid species, namely C. dentatus from Madeira, C. parasiticus from Ceylon (two forms), and C. svbpvbescens from Singapore. Interspecific hybrids have been set up between certain pairs of tetraploid species, namely C. svhpubescens (Singapore) x C. sumatranus (Singapore); C. sumatranus (Singapore) x C. jacuhsus (Ceylon); C. jaculosus (Ceylon) x C. subpubescens (Singapore). In morphology these F1 hybrids both triploid and tetraploid showed complete dominance in the expression of some of the morphological characters in which the parents differ and an intermediate condition in others. The more important characters showing dominance were: creeping rhizome versus erect rhizome, presence of anthocyanin pigment on the rachis versus absence of it, uniform length of the lower pinnae versus gradual reduction in length, presence of glands versus absence of these. The more important characters in which simple dominance was not displayed (i.e. F1 hybrids were intermediate between the two parental types) were: depth of cutting of the pinnae, number of anastomosing veins at the base of the pinna segments (in old plants, cf. footnote to p. 737), length of hairs on the lower surface of the stipe. All the hybrids obtained, both triploid and tetraploid, showed a high proportion of aborted spores consequent on irregular meiosis. This was clearly evident even in cases where a preponderance of recessive characters in one parent resulted in a hybrid showing a very close morphological resemblance to the other parent. Spore sterility would therefore be an effective means of detecting hybridity in a herbarium specimen. Chromosome pairing at meiosis has been investigated in all these hybrids in a preliminary way. All show numerous unpaired chromosomes as one factor contributing to spore abortion. In addition, the triploid between C. repandvlus (Ceylon) and C. dentaius (Madeira) showed few or no pairs, indicating rather remote relationship between these two species. The other triploids involving C. repandvlus with C. parasiticus and C. subpubescens respectively all showed approximately equal numbers of pairs and univalents, suggesting that the repandvlus genome is represented in each of the two tetraploid species as half the nuclear content. Among the tetraploid hybrids pairing approximating to n pairs and 2n univalents was found in each of the two combinations C. subpubescens x C. sumatranus and C. sumatranus x C. jaculosus, suggesting that in each of these pairs of species a diploid ancestor is shared. This ancestor could be C. repandvlus in one or both cases, though the evidence is insufficient to determine this. Present knowledge of the ecology and geographical distribution of C. repandvlus is outlined and a method is suggested for searching for the possible existence of other diploids not yet identified. A comparison is made between the C. parasiticus complex in the oriental tropics and the situation already partially elucidated by other workers on the Dryopteris spinvlosa complex in Europe. The resemblance is so close that it seems likely that there are factors operating in tropical floras which are at least as powerful though necessarily somewhat different in kind from those associated with the succession of glacial and inter-glacial conditions which have so strongly influenced the evolution of floras in temperate regions.