This paper reviews the current status of nematodes with stress-inducible transgenes as biosensors responsive to a range of external stressors, e.g., soil or water pollution, microwave radiation or immunological attack. Transgenic Caenorhabditis elegans carrying reporter genes under heat shock promoter control express reporter products only under stressful conditions. Although relatively insensitive to single metal ions, these worms respond to complex mixtures present in metal-contaminated watercourses and to laboratory mixtures containing similar constituents, but not to any of their components singly at comparable concentrations. Responses to metal mixtures are enhanced by a non-ionic surfactant, Pluronic F-127. Metals taken up by food bacteria and insoluble metal carbonates can also evoke stress responses, both in soil and aqueous media. However, high concentrations of added metals are needed to induce clear-cut responses in soil, owing to metal sorption onto clays and organic matter. Transgenic worms are also stressed by exposure to microwave radiation; pulsed signals generate responses that diminish markedly with distance from the source. Finally, stress responses are inducible by anti-epicuticle antisera and complement, suggesting that immune attack can also activite the heat shock system. The development of rapid microplate toxicity assays based on transgenic nematodes is discussed.