The majority of homosporous ferns have a chromosome number that is severalfold greater than that of diploid seed plants. These fern species have therefore been generally considered to be of polyploid origin. Enzyme electrophoretic investigations have demonstrated, however, that within fern genera, species having the lowest chromosome numbers (n = 27-52) have the number of isozymes typical of diploid seed plants; there is no isozyme evidence for polyploidy of these plants. We have constructed a genomic DNA library from Polystichum munitum (n = 41), a homosporous fern, and have screened the library for sequences homologous to the chlorophyll a/b-binding (CAB) protein genes of higher plants. The majority of the sequences isolated and characterized by nucleotide sequence determination represent defective CAB genes. This result is in contrast to the situation in the genomes of diploid angiosperms, where most, and sometimes all, copies of the CAB gene family represent functional members. Several hypotheses could explain the existence of multiple defective CAB genes in P. munitum. (i) The defective CAB genes are the result of "gene silencing" following polyploidy. (ii) P. munitum has not gone through a polyploidization event, but several, and perhaps the majority, of its CAB genes have mutated to a nonfunctional state (a phenomenon not yet observed in any of the genomes of non-fern plants so far examined). (iii) Some defective CAB genes have been specifically amplified in the genome of P. munitum.