BACKGROUND:  In China, mushrooms are often cultivated on cottonseed hulls, and spent substrates from their cultivation are used as cattle feed.  As the area planted to Bacillus thuringiensis (Bt) cotton in China increases, hulls from transgenic Bt cottonseed are being increasingly used as a substrate for edible mushroom cultivation.  The objective of this research was to determine whether Bt toxin was transmitted to mushrooms cultivated on transgenic Bt cottonseed hulls or retained in the spent substrate.  RESULTS:  Bt toxin was still present in transgenic Bt cottonseed hulls after chemical (carbendazol) treatment but not after heat (100 °C) treatment before mushroom fungus inoculation.  When mushroom was cultivated on carbendazol-treated substrates, Bt toxin level in the substrates fell below the detection limit (1.5 ng Cry1Ac per g fresh weight) in 15 days.  CONCLUSION:  Bt toxin was not transmitted to mushrooms during cultivation on transgenic Bt cottonseed hulls.