Apparent biomagnification factors (BMFs, wet weight basis) for organochlorine compounds in herring gulls (Larus argentatus) in Lake Ontario were shown to be related to chlorine substitution patterns in the case of polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCBs accumulated according to the availability of adjacent positions not substituted with chlorine: (a) para-meta unsubstituted (P group), mean BMF 20 ± 6.2; (b) meta-ortho unsubstituted (M group), mean BMF 87 ± 36; and (c) no adjacent unsubstituted positions (blocked, B group), mean BMF 154 ± 39. Among the other organochlorines, DDT, cis- and trans-nonachlor, dieldrin and octachlorostyrene magnified to the least extent (BMF 3-19), and photomirex, mirex and DDE magnified to the greatest extent (BMF 85-100). Of the PCDDs and PCDFs, 2,3,7,8-TCDD and 2,3,4,7,8-PnCDF biomagnified to a greater degree than did other congeners (BMF 32 and 7, respectively). There were no significant differences among egg-to-whole-body ratios (mean 0.60 ± 0.11, lipid weight basis) for any of the organochlorines except PCDDs and PCDFs, indicating that partitioning among lipid pools in the body is the main determining factor in tissue distribution. Egg-to-whole-body ratios for PCDDs (mean 1.2 ± 0.33) and PCDFs (mean 0.87 ± 0.02) were higher than but not significantly different from those for other organochlorines. Liver-to-whole-body ratios (mean 0.80 ± 0.16) also did not differ significantly among organochlorines, except that heptachlor epoxide, dieldrin, PCDDs and PCDFs had higher ratios. The mean fraction of the total body burden of PCDDs and PCDFs contained in the liver ranged from 5 to 55%, compared with 1 to 5% among the other organochlorines, and it increased with degree of chlorination.