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Aquatic habitats with immature populations of the Culicoides variipennis (Coquillett) complex were sampled across the United States and tested for soil chemistry parameters. Indicators of organic loading, phosphate, percentage of organic matter, and nitrate were high for C. variipennis and C. sonorensis, reflecting the common occurrence of larvae in sediments contaminated with livestock waste; these factors were low in western playa lake habitats with immature C. occidentalis. Levels of salt-forming ions and indicators of salinity, notably sodium, chloride, magnesium, sulfate, boron, and conductivity, were elevated in substrates supporting all 3 species, with values that declined from very high to high across the arid west to moderate east of the Mississippi river, and differed significantly among species. Concentrations of boron differed most strongly among habitats of the respective species, ranging from exceptionally high values in playa lake substrates with C. occidentalis, to high and moderate levels in western and southeastern sites with C. sonorensis, to moderate levels for substrates in the east with C. variipennis. The presence of C. sonorensis in dairy wastewater lagoons and saline springs in geographic regions that are otherwise inhabited by C. variipennis may reflect salinity factors that regulate the distribution of the C. variipennis complex.