This study endeavored to test two hypotheses: (i) Hyalorbilia oviparasitica and related species (H. oviparasitica clade) will be detected in sugar beet fields in California’s Imperial Valley and (ii) the population densities of these fungi will increase in the presence of their nematode host (Heterodera schachtii) and a host plant of this nematode. The H. oviparasitica clade includes potent hyperparasites of sedentary stages of plant-parasitic nematodes, which have the ability to substantially suppress the population densities of several endoparasitic nematode species. In this study, 21 of 25 Imperial Valley soils cropped to sugar beets harbored members of the H. oviparasitica clade. The population densities of these fungi increased, on average, approximately 10,000-fold in the presence of H. schachtii and its host Swiss Chard over one nematode generation. An rRNA ITS sequence analysis showed that members of the H. oviparasitica clade were the dominant group of fungi associated with H. schachtii females derived from these soils. These results provide evidence supporting the hypotheses. [Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .