Environmental concerns of potential contamination of ecosystems with radionuclides from nuclear power plants are becoming increasingly important. This concern makes it important to study the effects that radiation may be having on chromosomal damage in non‐human biota. This can lead to a better understanding of the potential effects of radiation on aquatic organisms (i.e. radiation sensitivity), and by extension, the impact on ecosystems. A common technique for determining chromosomal aberrations is the cytokinesis‐block micronucleus (MN) assay. This assay blocks cell division prior to cytokinesis, allowing detection of DNA damage or improper chromosome segregation. This DNA damage then results in the formation of micronuclei. Micronuclei are similar to the cell's main nucleus but are smaller and may contain either whole chromosome pieces or fragments of a chromosome. We hypothesize that the frequency of micronuclei in fish will be proportional to the exposed dose. Studies have also shown that some cells are capable of an adaptive response when exposed to a low priming dose prior to a large challenge dose of ionizing radiation. It is believed that the priming dose stimulates various cellular systems including DNA repair processes, making the cells more resistant to the larger dose of ionizing radiation. There is little data concerning the adaptive response in fish cells. Our preliminary experiments have determined that Chinook Salmon are significantly more radiation resistant than mammalian cells. Adaptive response experiments are in progress. We will also report on the sensitivity of low dose ionizing radiation in other fish species.