The purpose of this study was to assess what impacts on organized mosquito control the implementation of an Endangered Species Protection Program for the Houston toad might have in Chambers and Harris counties, Texas. The study was also intended to demonstrate the value of using geographic information system (GIS) techniques and methodologies in making such assessments to those in mosquito control who are unfamiliar with GIS and its applications. Using the GIS, Geographical Analysis Support System (GRASS), databases were developed on the habitats and patterns of mosquito control insecticide usage occurring in Chambers and Harris counties. These databases were then employed by means of various utilities associated with GRASS and computer-supported, rule-based reasoning processes to create maps depicting the amount and locations of toad habitat and the areas treated annually with insecticides by districts in Chambers and Harris counties. This map information was then used via other GRASS utilities to identify and depict zones of overlap or coincidence between toad habitat and areas treated with insecticides for mosquito control in the 2 counties. As compared to existing maps for toad habitat, our resulting GIS-generated maps gave more precise, easy-to-use information that could be used to make decisions as to how to protect the toad in the zones of coincidence in each county without causing undue disruption to mosquito control activities in these zones.