Interactions between species contribute to macro- and microevolution as well as to community structure and functioning. In addition to direct predator-prey relations, many other interactions (competition, facilitation, inhibition, etc.) may be key to the structure and functioning of communities. Such interactions often occur between species belonging to the same trophic level, and have already been demonstrated between species of bacterial-feeding nematodes, but the exact mechanisms remain poorly understood. Hence, iris difficult to predict their outcome under changing environmental conditions like pollution. Scientists and policy makers have come up with standardized tests to establish environmentally acceptable levels of pollution. In this experiment, we demonstrate how sublethal levels of contaminants can affect not only the fitness of two nematode species but also the way in which these species interact. We performed monospecific and combined culture experiments with two species of soil bacterivorous nematodes, Acrobeloides nanus and Plectus parvus, under optimal conditions and compared the outcome with trials in which the nematodes were exposed to a series of sublethal cadmium concentrations. By evaluating fitness and outcome of interspecific interactions based on their abundance, our results (a) confirm the considerably higher tolerance of A. nanus than that of P. parvus to Cd: (b) demonstrate mutual facilitation between both species: (c) highlight that differential tolerance to pollution between bacterivorous species may affect the balance and outcome of their interactions leading to counterintuitive results such as a higher population fitness of one species (A. nanus) under increased pollution. (C) 2012 Elsevier B.V. All rights reserved.