A strain of Spodoptera littoralis Boisduval, selected for resistance to the Bacillus thuringiensis CryIC toxin, was used to analyze the inheritance of resistance. This strain resulted from a laboratory selection with spore-crystal preparations of B. thuringiensis CryIC toxin. At generation 12, resistance was >500 times the resistance ratio of the control. Dose-mortality curves of the resistant population, the susceptible strain; the F-1 and the backcrosses from reciprocal crosses were compared. The resistance was partially recessive. F-1 offspring from resistant males and susceptible females were more resistant than those from susceptible males and resistant females, indicating a clear paternal effect. Segregation of resistance was investigated in the 8 possible backcrosses between F-1 and one of the parental strains. Dose-mortality responses significantly differed from those expected under the hypotheses of a major resistance gene that was either autosomal or sex linked. It was concluded that resistance was most probably polyfactorial. These results are discussed in relation to the nature of bacterial toxin resistance genes in various insects.