Controlled release pesticides (CRP), also called encapsulated pesticides, are a relatively recent development, although controlled release technology has been in use by the drug industry for many years. The objectives of this study were to determine the effect of selected CRP's on fathead minnows Pimephales promelas when compared with that of the technical grade pesticide used in their formulation, to determine water solubility and half-life in Lake Superior water, to perform a rough qualitative analysis of degradation products and to develop test methodology for short- and long-term CRP studies.