The 'Wijcik' mutant of the apple cultivar 'McIntosh' shows columnar-type growth, characterized by altered branching and a thick stem. Instead of normal branches, spur-like branches are produced on 'Wijcik' trees, leading to apples produced close to the stem. Columnar-type growth is a desirable trait, as columnar apple trees need considerably less pruning compared to standard trees. Furthermore, apple picking and the application of chemicals is much easier on columnar trees. The genetic basis for columnar growth is a somatic mutation that results in dominant inheritance of the columnar growth trait. The columnar gene (Co) has been previously mapped on apple linkage group 10. Three adult segregating populations, with a total of 301 F1 plants, and two three-year old populations of 1,250 F1 plants were used for fine mapping of the Co region, using newly developed simple sequence repeat (SSR) markers that were designed based on the 'Golden Delicious' apple genome sequence. Co was found to cosegregate with SSR marker Co04R12 and the Co region was found to be delimited by SSR markers Co04R11 and Co04R13 corresponding to a genetic region of 0.56 cM and a genomic region of 393 kb in the 'Golden Delicious' genome. By using BAC libraries, we could identify the clones covering the Co region in 'McIntosh' and 'Wijcik'. This enabled us to sequence the complete Co region in both cultivars. Sequence comparison between 'McIntosh' and 'Wijcik' lead to the identification of a 1,956 bp insertion in 'Wijcik'. This work resulted in the development of a perfect marker for columnar growth for application in breeding, and, in the future, will result in better understanding of the physiological background of columnar growth and the availability of the Co gene for biotechnological applications to manipulate tree architecture.