Spore viability and larvacidal activity of B. sphaericus 2362 and 1593 strains were monitored in media containing Gramaxone(R) herbicide. Minimal Inhibitory Concentrations (MICs) of the herbicide were determined for each of the mosquito pathogenic strains. Addition of paraquat into the media suppressed the spore germination after 24 and 48 h, but no influence was observed on parasporal inclusions containing insecticidal toxin proteins. SDS-PAGE analysis confirmed the presence of the so-called binary toxin comprising two proteins that are deposited in parasporal crystals treated with the herbicide. Moreover, the number of heat-resistant spores and total cell numbers of test strains were reduced drastically when test strains were grown in media containing 50, 100 and 200 mg ml(-1) of paraquat. In contrast, there were no differences between heat-resistant spore counts of both strains when grown under diet concentrations of less than 50 mg ml-1. This study revealed that B. sphaericus 2362 strain has a higher mosquito larvacidal activity than 1593 strain in the presence of paraquat in the media. Thus, the use of Gramaxone(R) herbicide can have harmful effects on mosquitocidal biological control agents.