Responses of mature Mamestrae brassica larvae, in the fifth and sixth instars, to nuclear polyhedrosis virus (NPV) have been quantified. There was an 86 fold decrease in susceptibility to virus, related linearly to increasing body weight, for the larval weight range tested. Although the rate of decreasing response to weight was constant, three phases of susceptibility were identified by shifts in the positions of the regression line of log LD50 on log body weight. Larvae above 700 mg were proportionately less susceptible than those between 250 and 700 mg which were, in turn, proportionately less susceptible than those below 250 mg. LT50 responses were inversely related to virus dosage but increased with larval age. This was accompanied by an increase in the proportion of infected larvae that eventually died in the pupal stage. The ratio of LT50 to time remaining for larval development provided a good predictor of the time course of NPV infection for all larval stages. Extensive comparison with a previous study on responses of immature M. brassicae larvae to NPV infection (6) have been included.