not in Young The hemocytes participate in the cellular encapsulation around foreign material implanted into the insect hemocoel. However, the suppression of this reaction by entomopathogens remains largely misunderstood. This study describes the ultrastructural modifications in one type of hemocyte of Diatraea saccharalis larvae, the oenocytoid (OE), along the parasitism by the wasp Cotesia flavipes, using conventional and cytochemical techniques. The OE of parasitized D. saccharalis larvae exhibited areas of cytoplasm rarefaction with intensity proportional to the time of parasitism. There was cell lyre at long-term parasitism, although we did not observe melanization either into the host haemocel or around of parasitoid. The OE of either control or parasitized larvae did not show phenoloxidase activity in the ultrastructural preparations; these absence was related with the morphological modifications caused by the parasitism. The trimetaphosphatase and acid phosphatase activities we! Diatraea saccharalis (Pyralidae): Host; Cotesia flavipes (Braconidae): Parasite. Cotesia flavipes: Lepidopteran hosts, Diatraea saccharalis, Host larval oenocyte ultrastructural and cytochemical response; Diatraea saccharalis: Enzymes, Acid and trimetaphosphatases activity, Haemocytes, Larval oenocyte, Hymenopteran parasites, Cotesia flavipes, Larval oenocyte ultrastructural and cytochemical response. ST Insecta, Lepidoptera, Glossata, Heteroneura, Pyraloidea, Pyralidae; Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Braconidae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans. none