Bioconcentration factors, uptake rate constants (k1), and clearance rate constants (k2) were determined for bromo- and chlorobenzenes with the mosquito fish (Gambusia affinis). The values of k1 and k2 on a lipid weight basis ranged from 3500 to 47,000 d−1 and 1.50 to 0.27 d−1, respectively. The rate constants exhibit good nonlinear correlations with the n-octanol/water partition coefficient (Kow). The bioconcentration factors reach a maximum at pentachlorobenzene (log Kow = 5.2) and then decline. The bioconcentration factors on a lipid weight basis (KBL) correlated well with Kow. The bioconcentration kinetics can be described by equivalent mass transfer models and experimental data indicate that the mass transfer process in the lipid phase is the rate-limiting step in the bioconcentration process.