Synthetic cecropins, antibacterial peptides from insect haemolymph, have been tested for their ability to attenuate the motility of microfilariae of the filarial nematode Brugia pahangi in an in vitro assay. Fifty micromolar concentrations of these peptides, equivalent to physiological concentrations in immune-stimulated insects, cause significant attenuation of motility compared with untreated microfilariae. Similar results were obtained with cecropins A and B. This is the lowest concentration for which cecropin has been reported to be active against eukaryote organisms. Antiserum to the cecropin homologue sarcotoxin 1A successfully blocked the observed activity. When the same concentration of cecropin B was coinjected with B. pahangi microfilariae into adult females of the mosquito, Aedes aegypti, a significant reduction in the numbers of developing larvae was observed.