Perennial low milkfish yields from underutilized brackish water ponds in the Philippines result primarily from the low fertilizer application rates of most farmers. The purpose of this study, based on a 1981 survey of 447 milkfish farmers in four climatic zones and on data from a previous survey of 324 farmers, was to determine constraints on more intensive fertilizer application. Of 56 socioeconomic, institutional, physical, and biotechnical variables tested, 8 were found to explain 73% of the variance in fertilizer use. The most significant variables were the ratios of milkfish price to organic and inorganic fertilizer prices, farmers' interest in working on other milkfish farms, and farmers' opinions regarding the effect of fertilizers on the taste of milkfish, followed by pond soil salinity, farmers' interest in seeking consultation, family size, and farmers' estimate of a fair collateral requirement for loans. It was concluded that milkfish farmers are responsive to relative prices of inputs and outputs and will adjust their fertilizer expenditure accordingly; high costs of credit and of organic fertilizers in some locations coupled with declining real milkfish prices often inhibit increased fertilizer use. The authors conclude that strengthened institutional support is necessary to accelerate the milkfish industry's ongoing transformation to higher yields and recommends a dual fertilizer pricing policy, increased production credit, and intensified extension services. Included are 48 tables and figures, 35 references (1962-83), and a copy of the survey instrument. (Author abstract, modified).