The planarian Schmidtea mediterranea has extraordinary regenerative capacities. This long-lived non-parasitic flatworm actually reproduces by binary fission, followed by the replacement of missing body parts. The large populations of adult stem cells needed to achieve this feat make S. mediterranea an ideal model in which to study adult stem cell biology. Bret Pearson and Alejandro Sánchez Alvarado now report that the single planarian p53 homolog (Smed-P53) regulates proliferation and self-renewal in adult stem cell lineages in S. mediterranea (see p. 213). They show that Smed-p53 is predominantly expressed in newly made, postmitotic stem cell progeny and that RNAi knockdown of Smed-p53 initially leads to stem cell hyperproliferation, which indicates that Smed-p53 has a tumour suppressor function. However, ultimately, in the absence of Smed-p53 expression, the stem cell population fails to self-renew. By showing that an ancestral p53-like molecule functions in both stem cell proliferation control and self-renewal, these results provide new insights into the evolution of the p53 family of transcription factors.