In the past, many landfill sites were constructed and operated without engineering containment, and from which leachate is likely to arise over the next several decades. Landfill leachate can contain high concentrations of toxic substances, which may pose a threat to the surroundings. In the worst cases, leachates may seep through the underlying substrata causing contamination of groundwater, of surface water drains, and ultimately of the rivers into which they discharge. As a consequence, the diversity and species richness of benthic communities are often degraded. The chronic toxicity of a landfill leachate was investigated in this study, with respect to determining the ‘environmentally safe’ concentration in which the long term survival of an Asellus aquaticus population is assured. The leachate was from a disused site known to contain industrial wastes, and samples were collected from a surface drain. The leachate used in the toxicity tests had a 600 mg l− 1 BOD5 and 1200 mg l− 1 COD. Sub-lethal toxicity tests were carried out in leachate concentrations that were lower than the acute toxicity threshold of Asellus aquaticus. The toxicity was judged based on birth frequency and the final length of juveniles. Tests showed that even a dilution of 1 : 20 would influence the breeding colony size of Asellus. A 30 mg l− 1 COD concentration was judged to be the ‘environmentally safe’ leachate dilution in which the frequency of births and juvenile length (after 4 weeks of monitoring) would not be affected. At this concentration, the integrity of an Asellus population would be protected.