In 115 mass-rearing cycles of Romanomermis culicivorax Ross and Smith, ratios of nematodes to mosquito larvae of 12, 13, 14, and 15.:1, produced mean levels of parasitism of 75, 80, 85 and 88%, respectively. Levels of parasitism between 51 and 60% produced an average 6.3 g of postparasitic nematodes compared with 10.9 g at levels of parasitism between 91 and 100%. Nematode cultures averaged 1912 x 103 preparasites if they were flooded for the first time when they were between 11 and 19 weeks old; hatch declined with age when the cultures were more than 19 weeks old at the time of the first flooding. Total yields were highest (5316 x 10 preparasites) when cultures were flooded for the first time when 8-10 weeks old and then at 3-4 week intervals thereafter.