The toxic effects of the antiandrogen cypermethrin on B. calyciflorus were investigated using the 2 d population growth rate (r) of B. calyciflorus. The effects of low-dose of cypermethrin (0.001-0.316 mg x L(-1)) on the 3 d population parameters, the 7 d resting egg production and the hatching rate of resting eggs of B. calyciflorus were also studied. The 2 d population parameters of B. calyciflorus were used to estimate the growth ability of larvae hatched from the resting eggs formed in cypermethrin. The 3 d population parameters were used to estimate the effect on the the reproduction of the offspring of B. calyciflorus, whose parent generation was preexposed in cypermethrin. The results indicated that the logarithmic concentration of cypermethrin had a linear negative correlation with the population growth rate. The half maximal effective concentration (EC50), the lowest observed effect concentration (LOEC) and no-observed effect concentration (NOEC) of cypermethrin were 14.22 mg x L(-1), 10 mg x L(-1) and 3.16 mg x L(-1), respectively. The resting egg production was decreased by 41.23% at 0.031 6 mg x L(-1) cypermethrin. The hatching rate of resting eggs was significantly decreased when formed in 0.031 6 mg x L(-1) cypermethrin. The population growth rate and mictic rate were significantly decreased for the individuals hatched from the resting eggs formed in cypermethrin. The 3 d population growth rate was significantly decreased by 15.96% compared to the control when their parent generation was pre-exposed in 0.316 mg x L(-1) cypermethrin. The results showed that the 2 d population growth rate of B. calyciflorus was less sensitive to cypermethrin. Low doses of cypermethrin reduced the resting egg production, the hatching rate of resting eggs and the population growth of hatched resting eggs, which would thus decrease the contribution of the offspring to the population growth.