Abstract. Two independently isolated 5-fluoro-2'-deoxyuridine (FUdR)-resistant mutant lines of Arabidopsis thaliana (L.) Heynh., FUD-1 and FUD-2, were identified by screening M2 populations of ethylmethane-sulfonatemutagenized seeds. The resistance was found to be due to single, recessive, nuclear gene mutations. Genetic complementation tests indicated that these two mutations were in the same gene locus, which was designated furl, and mapped to linkage group four of Arabidopsis. Enzyme assays indicated that the mutants were not defective in thymidine-kinase activity. Greatly reduced concentrations of intracellular 3H were detected in furl/furl plants compared with the wild type after incubation of wild-type and resistant plants in a medium with [3H]FUdR, indicating that either reduced uptake of FUdR or enhanced efflux of FUdR metabolites was the major reason for FUdR-resistance. furl/furl plants also had significantly decreased uptake of thymidine and uridine compared with the wild type but no difference was found in the uptake of adenosine, guanosine, thymine, uracil or amino acids. It is suggested that the transport system affected in the furl/furl mutants is one specific to pyrimidine nucleosides.