Part I. Literature and theoretical studies have demonstrated the likely existence of critical thermal transition regions for biological activity. Highly non-linear thermal effects, observed in many biological systems, appear to be manifestations of higher-order phase transitions. The origin of these transitions appears to be the vicinal water of the cellular systems. As these thermal effects are manifestations of intrinsic structural changes in vicinal water, the effects are likely invariants in terms of time and space. Thus, the corresponding critical temperature regions may represent absolute, upper permissible thermal pollution limits. Part II. Laboratory experiments, using some 18,000 individuals, have given the most accurate account of thermal tolerances for marine estuarine organisms to date. The organisms examined included the most important macro-algae and larval stages of important food-chain organisms. The expected Gaussian or skewed-Gaussian curve for lethal thermal limits did not materialize. Instead an abrupt death point occurred often within an interval of 1°C and in many cases within 0.5°C resembling a step function. One of the most important conclusions from this data is that the temperature tolerances obtained in the laboratory conformed closely to those observed in the field. Thus, the field data (Perkison, in preparation, Bader, et al., 1970) could be interpreted with more validity as to the effect of temperature versus other environmental factors. It should be emphasized that the laboratory upper thermal limits of the algae were borne out in distribution in the field in each case. The upper limits found in the laboratory for Halimeda, Penicillus, and Valonia were found to be the thermal limits in the field. When sustained temperatures above those found as laboratory survival limits were encountered at Turkey Point, these plants disappeared. In addition, detailed laboratory observations on the morphology of thermally stressed and thermally killed plants aided field observations of "effected" areas. The upper temperature limit for many of the plants examined as well as the sensitive stage of the pink shrimp, crab megalops and several carideans was 31 to 33°C. As previously stated, this was corroborated in field investigation where the mean annual temperature exceeded these limits near the mouth of the effluent canal at Turkey Point. These critical temperatures are within 1 to 3°C of mean mid-summer temperatures encountered under natural conditions. This substantiates the hypothesis that tropical marine organisms live closer to their upper lethal limit than do either temperate or Arctic species. This report (parts I and II) are submitted in fulfillment of project number 18050 DET contract under sponsorship of the Water Quality Office, Environmental Protection Agency. Part II of this work was partially sponsored by the U.S. Atomic Energy Commission.