The present study examined the effectsoftributyltin (TBT), polychlorinated biphenyls (PCBs), and mixtures ofboth chemicals on reproduction in Japanese medaka, Oryzias latipes. For 21 d we gave groups of medaka freeze‐dried brine shrimp flakes contaminated with a mixture of either 0, 1, 5, or 25 μg TBT g¹ plus 0 or 25 μg PCBs g¹. We measured the fecundity and fertility of the parent fish and assessed the deformity, hatchability, time‐to‐hatching, and swim‐up failure rate of the next generation. Fertilization success in the third week of the administration period was significantly decreased by administration of 25 μg TBT g⁻¹ (77%) compared with the control group (87%). Both TBT and PCBs were transferred maternally into the eggs of the next generation, causing early life–stage toxicity. Administration of 1 μg TBT g¹ was not toxic to embryological development, but abnormal eye development (i.e., small eyes or no eyes) occurred when TBT at the same concentration was mixed with PCBs (6.4%). Administration of TBT alone significantly decreased hatchability and increased swim‐up failure, and administration of PCBs alone significantly increased time‐to‐hatching. Statistical analysis by two‐way analysis of variance detected an interaction between TBT and PCBs in these three parameters. TBT induces abnormal development of the eyes, reduced hatchability, and increased swim‐up failure, whereas PCBs delay time‐to‐hatching. Administration of mixtures of TBT and PCBs has more adverse effects on the developmental stage of medaka than does that of each chemical alone.