The fractional radon retention by the bluegill has been followed over a 1000-day period, and is described by the equation F = 0.01t0.34. The data were compared to observations in mice that have a comparable bone mineraI fraction of 68%. The fish displayed a remarkably low radon retention at 1 day post-injection, due presumably to a smaller bone crystal size (the large amorphous calcium phosphate component). Radon retention by the bluegill increased much more rapidly with time after injection than did radon retention by the mouse. The 226Ra retention for bluegills was described by the equation R = 0.8t−0.15.