Various combinations of vertebrate and invertebrate tissue culture and microbiological media were utilized in an attempt to culture Romanomermis culicivorax (Mermithidae: Nematoda) in vitro. Most media were unsuitable and caused nematodes to become lumpy, vacuolated, and granular. Slow and limited growth and development of internal structures of the nematodes were obtained with variously supplemented Grace's tissue culture and Schneider's Drosophila media. In an enriched Grace's medium, development attained by the nematodes after 3-4 wk was comparable to 4-5-day-old parasites grown in vivo in the mosquito host, Culex pipiens. Two molts were observed in vitro. Maximum dimensions in vitro were 7.0-mm length and 87-μm width at the widest point. The stichosome, stichocytes, and trophosome developed prominently. A filiform tail and highly cuticularized tube persisted throughout the culture period in vitro.