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