The cell line (BTI-TN-5B1-4) of Trichoplusia ni had been selected by Bacilllus thuringiensis (Bt) toxin Cry1Ac for 86 generations continuously. A high level of resistance was achieved and the partial characterizations of the resistance-selected BTI-TN-5B1-4 cells were studied in order to explore the mechanism of the resistance to Bt toxin. The cells selected by Cry1Ac had low level of cross-resistance (19.7 fold) to Cry1C, their ability of tolerance in the hypotonic solution increased evidently (2.5 fold), and there were differential membrane proteins between resistance-selected and unselected susceptible cells on two dimensional electrophoresis maps. We supposed the changes of membrane proteins in resistant cells would be linked to the cross-resistance to Cry1C and the increase of tolerance in hypotonic solution.