This study was conducted to examine the genetic factor causing genetic instability of peach eye color inheritance in Drosophila mauritiana strain. This D. mauritiana strain was crossed with GB-1(w(pch) allele) and Z8(mosaic) factor was suppressed by a genetic factor. This allele was designated as a fw(pch). The result of Southern hybridization analysis revealed that a specific 5.71 kb BamHI-HindIII fragment containing mariner was not observed in genomes of dark mosaicism (group B), but in fw(pch) and faint mosaicism fly (group A I). It might be suggested that the 5.71 kb restriction fragment containing mariner could be associated with the suppression effect of the Mos factor activity.