A major problem in cloning large segments of Plasmodium falciparum DNA is the instability of this very A + T-rich DNA in Escherichia coli. We have therefore investigated whether P. falciparum DNA segments over 100 kb in size cloned in the yeast Saccharomyces cerevisiae as yeast artificial chromosomes (YACs) are stable. An analysis of EcoRI fragments adding up to 230 kb, or nearly 1% of the P. falciparum genome, showed that sequences cloned as YACs were indistinguishable from the DNA in the P. falciparum genome, showing neither deletions nor rearrangements. Over 400 YACs have been obtained so far with an average insert size between 110 and 130 kb. Screening the YACs with single-copy probes suggested that most sequences should be represented in a library of less than 10(3) YACs. Hence YACs provide an approach to cloning large segments of P. falciparum DNA.