The lower British Columbia mainland waterways are inhabited by two races of the prickly sculpin Cottus asper Richardson. One race lives in the lower part of rivers or in estuaries and spends at least part of its life, particularly during the spawning season, under tidal influence. This race is the "coastal". The other race, called "inland", is confined entirely to fresh water where it is found in scattered, more or less isolated populations. By investigating the effects of ambient salinity on the oxygen consumption and on the utilization of injected radioiodine, it was found that salinity profoundly effects the iodine metabolism and, to a lesser degree, standard metabolic rates of the fish. In fresh water the thyroids of both races appear relatively quiescent but with increasing salinity, particularly in hyperosmotic environment, there is a marked increase in thyroid uptake of tracer iodine and also in the levels of circulating thyroid hormone. The inland fish demonstrate a greater ability than coastal fish to retain iodide in fresh water, and in sea water their thyroids appear more active. The inland fish also have a lower oxygen consumption in fresh water than do coastal fish. Viewed together, the results lead to the conclusion that the inland fish, with respect to their osmoregulating ability, show genetic divergence from the coastal forms. The latter probably more closely resemble the ancestral form. / Science, Faculty of / Zoology, Department of / Graduate