The shrub Phyllanthus piscatorum Kunth (Euphorbiaceae) is cultivated by various ethnic groups of the Amazon because of its piscicidal properties. During ethnobotanical fieldwork among the Yanomamï Amerindians in Venezuela we observed that Phyllanthus piscatorum was exclusively cultivated and used by the women. Aerial parts of this herbaceous shrub are employed as fish poison and medicine to treat wounds and fungal infections. In addition, the leaves are used as tobacco substitute. Ethnobotanical data regarding the context of the use of this plant are presented. To validate ethnobotanical information related to its medicinal indications, antimicrobial, and antiprotozoal properties of water, methanol (MeOH) and dichloromethane (DCM) extracts were studied. No activity against Gram-positive bacterial strains but significant activity against the fungi Aspergillus fumigatus, Aspergillus flavus and the yeast Candida albicans were found. All extracts showed weak in vitro activity against Plasmodium falciparum and Trypanosoma brucei rhodesiense. The extracts were further investigated for cytotoxic effects in an in vitro test system with leukemia Jurkat T, HeLa, and human peripheral mononuclear blood cells (PBMCs). During the first 48 h the extracts did not exhibit any cytotoxicity. After 72 h the DCM extract potently inhibited viability of HeLa cells. Although in several communities along the upper Orinoco the cultivation and use of Phyllanthus piscatorum is being lost because of the ongoing acculturation, the traditional medicinal use of Phyllanthus piscatorum might provide an effective and cheap remedy against dermatological diseases linked with Candida albicans infections.