Platyhelminths possess a unique stem cell system that is claimed to be totipotent. It is supposed to be competent for the renewal of all cell types, including germ cells, during postembryonic development and regeneration. A connection to stem cells in the embryo has been postulated repeatedly. This cell type is now most frequently termed “neoblast”. Light microscopy can reveal only a few neoblast characters, and ultrastructural studies have shown additional characters for discriminating possible types andlor stages. While some progress has been made in this respect for triclad turbellarians (freshwater planarians), rather little is known about the microturbellarians. We have investigated the fine structure of neoblasts of hatchlings and adults ofMacrostomum hystricinum marinurn, a member of a primitive taxon in the “Turbellaria”-Rhabditophora (a paraphyletic group giving rise to the main parasitic flatworm taxa). InMacrostomum, one population of neoblasts is located in lateral bands along the main longitudinal nerve cords within the body cavity. Another population is found in the gastrodermis in a basi-epithelial position. Based on their cytoplasmic and nuclear organization, three stages in neoblast differentiation have been distinguished. The first and second stages are characterized by cytoplasm lacking organelles except free ribosomes and scattered mitochondria, a finding identical with the picture known from the “classical” planarian neoblast. In the first stage, heterochromatin is scattered over the nucleus in isolated clumps in a typical speckled (checkerboard) appearance; a nuclear lamina is weakly developed. In stage 2 the heterochromatin forms strands and clumps connected to each other. In stage 3 the nucleus is characterized by more prominent heterochromatin strands and by heterochromatin attachments to the well developed nuclear lamina. In this last stage a rough endoplasmic reticulum (rER) and Golgi complex are also present, indicating the entrance into cytoplasmic differentiation. Early epidermal replacement cells are located baso-epithelially, which show a nuclear organization similar to stage 3 neoblasts. Observations of stem cells in regenerating specimens and on isolated neoblasts are reported briefly. The data show that from the three types of differentiating cells distinguished recently in regenerative blastemas of planarians, the first stage (“undifferentiated cells”) resembles stage 2 neoblasp described here for postembryonic development. The results are compared with observations that have been published for neoblasts in other free-living platyhelminths.