Two resistances to downy mildew derived from Lactuca serriola were characterized genetically and mapped using molecular markers. Classical genetic analysis suggested monogenic inheritance; however, the presence of multiple, tightly-linked genes in each case could not be eliminated. Therefore, they were designated resistance factors R17 and R18. Analysis with molecular markers known to be linked to clusters of resistance genes quickly revealed linkage of R18 to the major cluster of resistance genes and provided six linked markers, three RAPD (Random Amplified Polymorphic DNA) markers and three codominant SCAR (Sequence Characterized Amplified Region) markers. The mapping of R17 required the screening of arbitrary RAPD markers using bulked segregant analysis; this provided five linked markers, three of which segregated in the basic mapping population. This demonstrated loose linkage to a second cluster of resistance genes and provided additional linked markers. Two RAPD markers linked to R17 were converted into SCARs. The identification of reliable PCR-based markers flanking each gene will aid in selection and in combining these resistance genes with others.