The influence of high magnetic fields was tested on the larvae of the American oyster, at the period of their setting. Two field strengths (5.8 X 10 -3 and 3.5. X 10 /3 webers/m²), of two to four days of exposure, were employed in configurations either parallel or perpendicular to the earth’s gravitational field. It was found that the magnetic field at the higher level resulted in 100% larval mortality while perpendicular to gravity, both at three and four days of exposures. The same field induced a 93% mortality together with an approximate doubling of sets above the normal controls while parallel to gravity at the same exposure times. Shorter exposures to this field were without effects. The lower magnetic field resulted in 100% larval mortality while perpendicular to gravity at four days of exposure but was without effect at shorter exposure times. When parallel to gravity it affected only 20% mortality with a simultaneous doubling of sets at four days of exposure and had no lethal effect but still increased the number of sets at any shorter exposure time tested. It seems that oyster larvae are sensitive indicators of the presence of higher magnetic fields and that their life functions are influenced not only by the strength of field employed but also by its direction. The underlying biological mechanism of this phenomenon is unknown.