A new cloned DNA probe (U6.2), which recognizes polymorphisms near the locus for the fragile-X syndrome, was isolated. No recombinations were observed between the probe and the disease locus, although recombinations were observed with several other probes known to be located close to the fragile site. The locus defined by the probe, DXS304, cosegregated with the fragile-X phenotype in 29 informative meioses (zeta = 4.97, tau = 0.00). The degree of polymorphism at this locus and its proximity to the fragile-X locus makes it useful for carrier diagnosis and as a new starting point for attempts to clone the gene responsible for the disease.