A study of Leptomonas lygaei in Lygaeus pandurus is described. Flagellates with promastigote configuration were found in the midgut, ileum and rectum. Cysts and encysting stages (straphangers) were found in the rectum either attached to the flagella of promastigotes or free in the lumen. In the posterior midgut, the crypts were frequently filled with flagellates, but no attachment to host epithelium was observed. A close association between the flagellates and extracellular membrane layers was observed. In the rectum the flagellates frequently attached by hemidesmosomes to the cuticle of the gland cells and less frequently to the rest of the rectal wall. Interflagellar desmosomes between the expanded sheaths of flagella of adjacent parasites were also observed. Straphanger cysts attached to parental flagella by the formation of zonular desmosomes. Differences were apparent between the organelles of the encysting stages and the parental flagellates. In mature cysts, the cellular organelles were unrecognizable. A comparison between the cysts and flagellates of L. lygaei and Blastocrithidia familiaris is made.