To determine the practicability of fertilizing trout lakes in Michigan, series of applications of inorganic, commercial fertilizers were made to four trout lakes which ranged in methyl-orange alkalinity from 138 to 192 p.p.m. Two additional lakes, left unfertilized, were used as controls. Increases in epilimnial phosphorus content, which occurred as a result of fertilization, were only temporary; there was no evident difference between total phosphorus concentrations in the fertilized lakes and the unfertilized lakes during the summer following 2 years of fertilizer applications. The rate of disappearance of added phosphorus from epilimnial waters was shown statistically to be related to alkalinity. Increases in total phosphorous concentrations occurred within the hypolimnia and, up to a certain point, were correlated with the alkalinity of the lakes. The theoretical lower limit of the alkalitrophic, or excess calcium, condition seemed to be between 120 and 160 p.p.m. methyl-orange alkalinity. The postulate that the ratio of marl to organic matter is important in the control of adsorption and regeneration of phosphorus in lakes was supported. It was assumed that this factor, as well as the descent of phosphorus from epilimnial to hypolimnial layers, accounted for the nearly complete disappearance of added phosphorus from epilimnial waters of the fertilized trout lakes.