An in situ system was developed for exposing benthic invertebrates directly to contaminated sediments without exposure to natural overlying waters. This system was used to expose fourth instar Chironomus riparius Meigen larvae for 48 h at 13 uncontaminated river sites across southeast England and at sites along the contaminated River Aire in Yorkshire. Recovery of surviving C. riparius from uncontaminated sites was good (>65%) except when indigenous predatory insect larvae were trapped within test systems. Recovery of survivors from the uncontaminated upstream site in the River Aire was also good (mean survival at Gargrave = 70%), but survival at the two contaminated downstream sites was significantly lower (mean survival of 17.5% at Esholt and 10% at Calverley Bridge). This study has shown that sediments can be isolated from overlying water and effectively bioassayed in situ.