The life history of Neoplasta parahebes MacDonald and Turner in a mountain stream in Southern California is reported. Female N. parahebes were separated from sympatric female Neoplasta hebes Melander by ovipositor morphology. Adults readily fed on adult midges (Diptera: Chironomidae) smaller than themselves. Ovarioles of gravid females contained an average of 74.8 eggs. Mating occurred in a unidirectional position. Females oviposited under the bark of submerged decaying wood. Eggs averaged 459 µm × 115 µm in size; most were laid within a 24 h period and, at room temperature, hatched in 10–11 days. There are three larval instars. Densities of larval N. parahebes were as high as 9.2 per 100 cm of wood surface in submerged dead tree branches. Branches also contained larvae of Orthocladius lignicola Kieffer, a wood-boring chironomid; larval N. parahebes readily fed on the midge larvae in their tunnels. Pupation occurred in the decaying wood.