Cystinuria is an autosomal recessive aminoaciduria in which three urinary phenotypes (I, II, and III) have been described. An amino acid transporter gene, SLC3A1 (formerly rBAT), was found to be responsible for this disorder. Mutational and linkage analysis demonstrated the presence of genetic heterogeneity in which the SLC3A1 gene is responsible for type I cystinuria but not for type II or type III. In this study, we report the identification of the cystinuria type III locus on the long arm of chromosome 19 (19q13.1), obtained after a genomewide search. Pairwise linkage analysis in a series of type III or type II families previously excluded from linkage to the cystinuria type I locus (SLC3A1 gene) revealed a significant maximum LOD score (zeta max) of 13.11 at a maximum recombination fraction (theta max) of .00, with marker D19S225. Multipoint linkage analysis performed with the use of additional markers from the region placed the cystinuria type III locus between D19S414 and D19S220. Preliminary data on type II families also seem to place the disease locus for this rare type of cystinuria at 19q13.1 (significant zeta max = 3.11 at theta max of .00, with marker D19S225).