In the study presented here we use flow cytometry to determine the differential hemocyte counts in partially suitable and unsuitable hosts (Diatraea grandiosella and Ostrinia nubilalis, respectively) parasitized by novel-association Cotesia spp. parasitoids around the time encapsulation occurred and compared results with those from the suitable host (Diatraea saccharalis). Flow cytometry was used since it is a proven technique that can rapidly classify individual cells based on their light scatter and fluorescence emission characteristics. The autofluorescence emission patterns of host hemocytes were analyzed, as well as emission patterns after labeling with florescein peanut agglutin (PNA). PNA selectively labels one of the cells thought to be important in the host’s immune response (granular cells), but does not label another important morphotye (plasmatocytes). We determined that flow cytometry is a useful method for studying the hemocytes of different lepidopteran species. Using this method we determined that there are clear differences in the proportions of plasmatocytes and granular cells depending on host species and developmental stage. However, differences in hemocyte proportions did not correlate with parasitization or the occurrence of encapsulation. In the future we will try to determine if the functioning of the hemocytes involved in the immune response is altered after parasitization of a suitable host and not after parasitization of an unsuitable host.