Background: Olyset nets are a type of long-lasting insecticidal net made of polyethylene fibre withpermethrin incorporated into it and with a 4 mm mesh size.Bioassays: Olyset netting was wrapped around a wire frame and the mosquitoes were released insidefor bioassays. There was significantly faster knockdown and higher percent mortality than bioassayswith the netting attached to a WHO cone with a piece of cardboard on the other side of the net toprevent escapes through the large mesh. It is suggested that with the latter method some mosquitoesplace their tarsae through the mesh on to the cardboard, thus avoiding insecticide contact.Trials in experimental huts: Four mm mesh nets were compared with conventional 1.5 mm meshnets treated with permethrin. In further trials in huts Olyset nets which were either unwashed or fivetimes washed, with or without subsequent heating, and a Olyset net which had been in domestic usefor four years or a new Olyset net were compared with a net treated with bifenthrin.Results & conclusion: In all cases Anopheles biting on sleepers under the nets was reduced andAnopheles mortality was increased by the use of the insecticidal nets. No significant impact of washingor heating was detected and an Olyset net was as good as new after four years use, but did not causeas much mosquito mortality as bifenthrin treated nets.