Hemocoelic inoculation of epimastigotes of Trypanosoma rangeli strain H14 into 5th-instar larvae of Rhodnius prolixus previously fed on blood containing the same parasites, showed reduced number of hemocyte microaggregates in the hemolymph, enhanced number of flagellates in the hemolymph as well as increased mortality of these insects. All these effects were counteracted by combined inoculation of R. prolixus with T. rangeli and arachidonic acid. In vitro assays using hemolymph taken from insects previously fed on blood containing parasites showed that hemocyte microaggregation reactions were also attenuated when T. rangeli is used as inducer of the reaction, and that simultaneous applying T. rangeli with arachidonic counteracted the hemocyte microaggregation inhibition. We suggest that arachidonic acid pathway can be a mediator of hemocyte microaggregation reactions in the hemolymph of insects inoculated with T. rangeli, and that oral infection with this protozoan inhibits the release of arachidonic acid.