Acute toxicity tests were conducted to determine the biological effects of partially purified extracts of four different plants on the juveniles of Culex quinquefasciatus. The active ethyl acetate extract of Solarium suratense leaf produced a 24h LC50 of between 23.53 ppm for the second instars and 46.04 ppm for the third instars. This was followed by the defatted methanol fraction of Abrus precatorius seed coat extract with the LC50 values ranging from 59.82 ppm for the second instar to 77.34 ppm for the third instar. The petroleum ether fraction of Solatium trilobatum and the ethyl acetate fraction of Lencas aspera were less toxic to the larvae. The active fractions of the plant extracts disrupted moulting and metamorphosis, induced malformation, extended the larval duration and inhibited adult emergence. At concentrations of less than 37.64 ppm, they inhibited emergence of 50% of the larvae. The insecticidal activity of these extracts persisted for up to four days.