The transplacental transfer of organochlorines (OCs) in California sea lions (Zalophus californianus) was investigated by analyzing blubber samples from 20 female sea lions and their fetuses during the last trimester of pregnancy. A rapid, high-performance liquid chromatographic, photodiode-array method was used to measure blubber concentrations of polychlorinated biphenyls (PCBs), including dioxin-like congeners, as well as DDTs and hexachlorobenzene. Summed values of PCBs (sigmaPCBs), of DDTs (sigmaDDTs), and of PCB toxic equivalents (sigmaPCB TEQs) were calculated from these data. The ratios of mean blubber concentrations of fetal sigmaPCBs to maternal blubber concentrations of sigmaPCBs were 0.45 by wet weight and 0.97 by lipid weight, but these ratios varied widely among mother-fetus pairs. Mean ratios of fetal sigmaDDTs to maternal sigmaDDTs were 0.53 by wet weight and 1.12 by lipid weight. Fetuses were classified into two age groups, based on date of recovery, to examine differences in OC transfer because of gestational age. Fetal to maternal ratios for individual PCB congeners, DDT compounds, and sigmaPCBs, sigmaDDTs, and sigmaPCB TEQs were lower among premature compared with late-term fetuses. These ratios increased for both groups as the logarithmic n-octanol/water partition coefficient (log Kow) for each compound decreased. Linear predictions for sigmaPCB and sigmaDDT concentrations in fetal blubber could be obtained using the sigmaPCB and sigmaDDT concentrations in maternal blubber, maternal and fetal blubber lipid content, maternal mass, and maternal age. Fetal TEQ was explained by maternal TEQ and maternal age. The ability to predict contaminant concentrations in fetal blubber from maternal parameters is important for developing risk assessment models for marine mammals.