Scanning and transmission electron microscopic techniques were used to determine the morphology of the exochorion and endochorion of Psorophora columbiae eggs as found in the ovary and after deposition. In the ovary, the outer chorionic reticulations and tubercles of the eggs are ornately developed below the follicular epithelium. Expansion of the eggs after deposition causes the outer chorionic reticuli and sculpturing (i.e., minor and major tubercles) to form the characteristic spinose appearance of Psorophora eggs. Transmission electron microscopic micrographs of cross and sagittal sections of the chorionic layers of Ps. columbiae eggs indicate that the major outer chorionic tubercles have 3 distinct regions. The 3rd region was found only on the dorsum of the tubercle. Elevation of major tubercles appears to be regulated, at least in part, by ambient moisture conditions, and one function of these tubercles seems to be adhesion of the egg to an oviposition surface.