Metallothionein (MT), a protein involved in metal regulation and detoxification, has been used widely as a biomarker of metal exposure. In the present study, a transgenic strain of the free-living soil nematode Caenorhabditis elegans was developed using the C. elegans MT-2 (mtl-2) promoter to control the transcription of green fluorescence protein (GFP) reporter. Response of this transgenic system to Cd, Hg, Cu, Zn, Ni, Pb, and As exposure in aquatic media was tested by quantifying GFP expression after 24 h of exposure. Response in Cd-spiked soil was tested in a similar manner. The mtl-2 transcription also was measured using real-time reverse transcription-polymerase chain reaction to gain a mechanistic understanding of the transgene expression. Green fluorescence protein is induced by Cd, Hg, Cu, and Zn in a time- and concentration-dependent manner; mtl-2 transcription is consistent with the GFP response. The minimum concentrations of Cd, Hg, Cu, and Zn that induce GFP response are 2- to 1000-fold lower than concentrations affecting traditional endpoints, such as lethality or behavioral change. The system responds to Cd in soil in a similar manner. Neither Ni nor Pb induces GFP, and neither induces mtl-2 transcription. Arsenic does not induce GFP, yet an increase in mtl-2 transcription was found, suggesting that As may interfere with GFP signaling. This mtl-2::GFP transgenic bioassay represents an alternative approach to quantify, both easily and quickly, a surrogate of MT in response to metal exposure (e.g., Cd, Hg, Cu, and Zn) in a variety of environments and potentially may be used for quantitative or semiquantitativebiomonitoring of metal contamination in soils and aquatic systems.