Bt maize varieties expressing the insecticidal protein Cry3Bb1 against larvae of the Western corn rootworm (Diabrotica virgifera virgifera; Coleoptera: Chrysomelidae) may harm non-target beetles due to the spectrum of activity of the protein. We have conducted studies to assess the prey-mediated effects of Cry3Bb1-expressing Bt maize (event MON88017) on the ladybird beetle Stethorus punctillum (Coleoptera: Coccinellidae). This species specifically consumes spider mites that are known to contain high amounts of Bt toxin when feeding on Bt maize. The developmental and reproduction life-history parameters did not differ for spider mites, Tetranychus urticae (Acari: Tetranychidae), fed Bt maize or non-Bt maize. Similarly, larval survival and development, adult survival, and adult dry weight did not differ for S. punctillum fed with spider mites reared on Bt or non-Bt maize for over 2 months. Female beetles that were fed T. urticae from Bt maize had a shorter pre-oviposition period, increased fecundity, and increased fertility relative to females fed spider mites from non-Bt maize. The reasons for these effects are unclear but may be due to unidentified differences in plant characteristics. T. urticae contained 56% of the Cry3Bb1 concentration in Bt maize leaves. Beetle larvae and adults that had consumed Bt maize-fed spider mites contained toxin concentrations that were six and 20 times lower, respectively, than Cry3Bb1 levels in their spider mite prey. Thus, the toxin was diluted at higher trophic levels. The results indicate that S. punctillum is not harmed by feeding on spider mites containing Cry3Bb1. Consequently, detrimental effects on this predator when preying in Cry3Bb1-expressing Bt maize fields are unlikely.