Three different concentrations of the antibiotic tetracycline in honey were tested for their influence on the offspring production and longevity of the parasitoid wasp Encarsia formosa. Several earlier publications did not provide a conclusive answer on the effect that the Wolbachia have on these wasps. The results of our experiments show that at high tetracycline hydrochloride concentrations in honey (50 mg/ml) the antibiotic is toxic to the females, all females died within three days after the antibiotic treatment. The concentration 5 mg/ml was less toxic although the treated females also lived shorter and produced less offspring than the control females. At the lowest tested concentration of 1 mg/ml there was no significant difference either in offspring production or in longevity between the control and the treated females. The antibiotic treatment at both 5 and 1 mg/ml resulted in exclusively male progeny after the first two days of oviposition. These results are consistent with the theory that in species in which all individuals are infected the Wolbachia should not impose a large fitness cost.