A statewide survey indicated that the mermithid nematodeHeleidomermis magnapapulaPoinar&Mullens was widespread in California. The nematode occurred in many manure-polluted habitats where the insect host,Culicoides variipennis(Coquillett), was found, but was absent from two saline or alkaline habitats. Nematodes emerged from larvae ofC. variipennisbetween February and November. Field-collected, late-instarC. variipenniswere held individually to determine the extent of parasitism. Parasitism ranged from 0 to 69% in southern California. Hosts parasitized by a single nematode most commonly yielded females, whereas hosts harboring multiple nematodes yielded males. The sex ratio was 2.4 males per female. Only three of 5,308 adult midges were found to be parasitized.