Congenital loco in chicks is characterized by an apparent lack of control of the muscles of the neck. This disorder is inherited as a simple Mendelian recessive disease, caused by an autosomal recessive gene, lo. To date, there are no reports on the localization of this gene. The objective of this study was therefore to identify the genomic region of the lo locus. The experimental congenital loco population used here were selected from a Rhode Island Red (RIR) line and consisted of six generations, resulting in 124 chickens. A total of 113 DNA samples from offspring of four generations (G3, G4, G5, and G6) were used for genotyping. At first, genome-wide linkage mapping was performed using 122 microsatellite markers on 22 autosomal chromosomes, and the lo locus was mapped to chromosome 12. We then performed fine mapping in two steps on chromosome 12. First, the lo locus was mapped to the interval between GGA12_5 and GGA12_11 using 13 new polymorphic markers. In the second step, fine mapping was performed by adding new families and 11 additional new polymorphic markers. Linkage mapping and haplotype information enabled the localization of the lo locus to a 1.1-Mb region between GGA12_28 and GGA12_30. Genetic markers between GGA12_28 and GGA12_30 may be used to remove the carriers of congenital loco through this RIR line.