Petroleum-ether and chloroform soluble fractions of methanolic extracts of mangrove/plants (Derris heterophylla, Salvadora persica, Sonneratia caseolaris, Clerodendron inerme), seaweeds (Acanthophora muscoides, Microdictyon pseudohapteron), seagrass (Halophila ovalis, Syringodium isoetifolium), and a lichen (Umbilicaria aprine) were studied for their toxicity against larvae of the mosquito Culex quinquefasciatus. Petroleum-ether fractions (PE) were more effective than chloroform fractions (C) in all the nine samples tested. The PE fraction of the seaweed Microdictyon pseudohapteron was found to be most effective with LC sub(50) values of 50 mg L sup(-1) followed by Acanthophora muscoides (62.5 mg L sup(-1)). The PE fraction of Syringodium isoetifolium was effective at 450 mg L sup(-1). All other ractions showed activity at between 800 and 2000 mg L sup(-1). The C fractions of Derris heterophylla, Sonneratia caseolaris, Acanthophora muscoides, Halophila ovalis, Syringodium isoetifolium and Umbilicaria aprine were ineffective even at 2000 mg L sup(-1)