Twenty-six pregnant big brown bats (Eptesicus fuscus) were collected at Montpelier Barn, Laurel, Maryland, and kept in individual cages until parturition. Seven young in 5 litters were born dead; 21 litters contained only living young. Polychlorinated bipbenyl (PCB, Aroclor 1260) crossed the placenta two to three times more readily than did DDE. Concentrations of PCB were significantly greater in litters with dead young than in litters where both young were born alive. The concentration of PCB in adult females plus their litters declined with increasing age of the female.