The rules governing proximo-distal regeneration in the wing disc of Drosophila were investigated. It was found that proximal fragments confined to either anterior or posterior compartments could not regenerate distally, although many fragments having tissue from both compartments could do so even in the absence of circumferential regeneration. Fragments containing the ventral but not the dorsal end of the anterior-posterior border were able to regenerate distally. The use of a cuticular marker in the posterior compartment very close to the border permitted precise localization of the tissue required to cause anterio fragments to regenerate distally; in anterior fragments cut close to the border, there was an almost perfect correlation between possession of this marker and distal regeneration. It was found however, that distal regeneration was not an all-or-none phenomenon; its extent was dependent on the total amount of tissue present from both compartments.