Bacterial insecticides play an increasingly important role in mosquito control. To establish guidelines for detecting resistance at an early stage, information on natural variation in susceptibility of insect populations to these insecticides is needed. Between 1990 and 1993, the susceptibility of Culex pipiens L. complex to Bacillus thuringiensis subsp. israelensis de Barjac and/or Bacillus sphaericus Neide was determined in 31 collections from California. These collections were undertaken before the widespread use of B. thuringiensis subsp. israelensis and before the registration of B. sphaericus in California. Seven collections from the Mediterranean island of Cyprus, where no microbial insecticides have been used, also were tested. The 1990–1991 California collections exhibited limited variation in susceptibility to B. thuringiensis subsp. israelensis. LC₅₀ and LC₉₅ values spanned about a three-fold and four-fold range, respectively. The 1993 Cyprus collections exhibited both higher mean LC values, and greater variability in those values, than the California collections. The LC₅₀s for the Cyprus collections varied over a 10-fold range, whereas the LC₉₅s varied over a 12.5-fold range. Variation in susceptibility to B. sphaericus among the 1991 California collections was about five-fold at the LC₅₀ and LC_95. No significant geographic variation in susceptibility to B. thuringiensis subsp. israelensis was observed among regions within California. Although variation in susceptibility was limited among California collections, the greater variability observed among the Cyprus collections and between the Cyprus and California collections illustrates the importance of establishing regional baselines to monitor accurately for changes in susceptibility.