The toxicity of two common organotin pollutants and their initial breakdown products (tributyltin, dibutyltin, triphenyltin and diphenyltin) were assessed using two different bioluminescent microbial biosensors: Microtox and lux-modified Pseudomonas fluorescens pUCD 607. The organotins were made up as standards, and tested both in double-deionized water and in extracted soil solution, the latter representing a realistic matrix for terrestrial contamination. Microtox was especially sensitive to the organotins, with 50% effective concentration (EC50) values (15 min) for tributyltin as low as 21.9 μg l-1 in pure water, and 0.118 μg l-1 in soil extract. The sensitivity of Microtox was increased by an order of magnitude in soil extract. The Ps. fluorescens was less sensitive, with EC50 values (30 min) of around 800 μg l-1 in pure water. The toxicity to Ps. fluorescens was decreased by around an order of magnitude in soil extract. The two biosensors showed different response patterns, with Microtox being more sensitive to the triorganotins and Ps. fluorescens being more sensitive to the diorganotins.