-6of these radioisotopes of todine is short (range from 2 hours to 8 days), the damage to the thyroids would be expected to occur soon after exposure. Because the genetic effects of lonizing radiations occur in the progeny of the exposed individuals and may be subtle, it cannot be said that mutations have not occurred in the fish in the Bikini-Eniwetok area, although it can be said that there have been no recognizable mutations in the thousands of fish that have been observed. If muta- tions have occurred they are not the type that manifest themselves as morphological abnormalities. Laboratory experiments rather than field observations are needed to determine the genetic effects of low, chronic doses of ionizing radiation. In the above discussion it has been pointed out that the indirect effect upon man of the uptake by fish of radioisotopes from the sea is of greater concern than the direct effect upon the fish themselves. presentation to follow, Therefore, in the principal consideration will be given to the role of fish in transferring radioisotopes from the sea to man. Because fish acquire radioisotopes either directly from the water, or indirectly by feeding