Larvae of Culex quinquefasciatus were exposed to infection by Lagenidium giganteum and various concentrations of B.t.i. or B. sphaericus. The resulting larval mortalities, percentages of infected dead larvae and percentages of larval body regions containing the fungus were compared. Overall, the effectiveness of Lagenidum giganteum against the larvae was not significantly affected by the presence of B.t.i. or B. sphaericus, and the fungal and bacterial agents were compatible. In experiments using 3-day-old larvae, the extent of growth of the fungus in the infected larvae and the percentage of the larvae infected were related to the concentration of B.t.i. in the range of 0.057-0.456 ITU/ml tested but were not related to the concentration of B. sphaericus in the range of 0.6-4.8 x 10(4) spores/ml. With larvae of various ages treated with a low concentration of B.t.i. (0.114 ITU/ml), exposure to the fungus increased the mortality rate in early but not late instars. After single and multiple applications of B.t.i. and B. sphaericus in the presence of the fungus, followed by drying and reflooding, the fungus persisted and reinfected larvae while the B. sphaericus persisted but the B.t.i. did not.