Abstract.Twoin vitrotests were used to investigate the effect ofOnchocerca lienalisStiles infection on the haemolymph ofSimulium ornatumMeigen. The first of these examined the effect of infected haemolymph on the motility of freshO. lienalisorBrugia pahangiBuckley&Edeson microfilariae. Incubation of haemolymph from individual flies with fresh microfilariae was performed in the wells of Terasaki micro‐tissue culture plates. Motility of both species of parasite was found to be significantly attenuated when compared to worms incubated in control haemolymph groups.The second assay was that of agglutination of cat erythrocytes in the presence of haemolymph from individual flies, also performed in Terasaki plates. This test demonstrated significant increases in the rates of haemagglutination in the haemolymph ofO. lienalisinfected blackflies. The titre appeared to increase during the initial 5 days of infection up to a level of 1/32+, but then fell between day 5 and 7 to a maximum level of 1/2. The proportion of flies exhibiting haemagglutination also rose following infection.Despite the apparent absence of melanization and encapsulation, simuliids may have at least two humoral haemolymph components available to them for parasite regulation; a fast‐acting factor responsible for rapid parasite death, and more specific agglutinins, possibly lectins. The role of the latter in defence is as yet uncl