Previous studies established that cytoplasmic incompatibility in the confused flour beetle, Tribolium confusum, is mediated by a maternally inherited rickettsia-like bacteria: crosses between infected males and uninfected females are sterile. All other crosses are fertile, and individuals can be cured of infection by treatment with tetracycline. We report that a third species-either actinomycete bacteria Streptomyces or fungi Penicillium-can cure beetles of infection with these parasites. Progeny from infected beetles were raised on flour produced from Streptomyces- or Penicillium-"molded" wheat grain. Microbial strains included known producers of tetracyclines and strains of related species that are commonly isolated from whole or milled grain. High rates of curing, eight of 10 males cured and nine of 10 females cured, were produced with grain molded with Streptomyces aurepfaciens (Northern Regional Research Laboratories [NRRL] 2209), a common soil-inhabiting, tetracycline-producing actinomycete bacterium. Low rates of curing were recorded for Streptomyces griseus (NRRL B-2249; 1/10 females), Penicillium verrocosum (NRRL 3712; 1/20 females), and Penicillium aurantiogriseum var. polonicum (NRRL 3704; 1/20 females). No curing was recorded for infected populations raised on eight other Streptomyces strains, 11 Penicillium strains, an autoclaved control, or brewer's yeast control. The high rate of cures from one strain of actinomycete and low rate from three other strains of fungi and actinomycetes suggest that local "patches" of antibiotic contaminated grain can promote a polymorphism of infection among Tribolium populations in nature.