Regenerative capabilities and mechanisms of posterior gut regeneration after bisection were investigated in Leptosynapta clarki using electron microscopy and scintillation counting experiments. Only anterior pieces containing the oral complex are capable of survival and regeneration. Using several ultrastructural characteristics of the lining epithelium of the stomach and intestine, it was found that there is a gradual transformation of the dominant cell type of the stomach to form the dominant cell type of the intestine. The transformation is accomplished in about 28 days. No increase in uptake of [3H]thymidine by regenerating tissues could be measured during the first 12 days of regeneration, and uptake by regenerating tissues did not differ significantly from those of nonregenerating control specimens. It was concluded that gut regeneration in L. clarki is accomplished by redifferentiation of stomach tissues directly into intestinal tissues without the formation of a blastema.