In the presence of second larval instars of three mosquito species the preparasites of Romanomermis culicivorax swam near the water surface in an orthokinetic manner. When the preparasites were ca. 1 mm from the host, they stopped and swam klinotactically toward the host. During this phase, the preparasites secreted a small amount of a putative adhesive material from the anterior region and host contact was completed. The adhesive appeared to aid in attachment of the preparasites to the host and initiation of the search-boring phase. The preparasites glided over the host until a suitable penetration site was found. The penetration phase was initiated by probing with the odontostyle. This was followed by partial paralysis, decreased intestinal peristaltic movement, and temporary cardiac arrest in all host mosquitoes which was probably related to injection of esophageal secretions. SEM observations showed that the abdominal walls were the most frequent site for penetration. As the preparasites entered through the penetration hole, microorganisms adhering to the cuticle of the preparasites were retained by the adhesive which accumulated around the penetration site. Thus, microbial contamination of the host was avoided by a mechanical cleansing mechanism. Penetration was usually completed in less than 10 min.