In October 2001 and March 2002, a field survey of central stonerollers (Campostoma anomalum) from perchlorate-contaminated streams in central Texas was conducted to assess thyroid endocrine disruption. A survey of adult male and female cricket frogs (Acris crepitans) was performed at the same site between 2001 and 2003. Perchlorate is an oxidizer primarily used in solid-fuel rockets, and many sites that processed or used perchlorate are now contaminated. Histological analysis revealed that the fish from contaminated sites had increased thyroid follicular hyperplasia, hypertrophy, and colloid depletion. Multivariate analysis was generally found to be more powerful than univariate analysis. Seasonal differences existed in the degree of thyroidal perturbation were discovered, and fish were generally less sensitive to thyroidal perturbations in March compared to October. Thyroidal histological indicators were also correlated to levels of perchlorate in the fish, water, and periphyton. Periphyton was frequently most strongly correlated to thyroidal indices, suggesting that exposure through the food chain may be of import. In addition, one of the presumed reference sites turned out to be contaminated with perchlorate, and this was reflected by thyroidal biomarkers before perchlorate was detected in the stream water or biota. There was no evidence of colloid depletion or hyperplasia in frogs from any of the sites, although frogs from two sites with greatest mean water perchlorate concentrations exhibited significantly greater follicle cell hypertrophy. Furthermore, there was a significant positive correlation between follicle cell height and mean water perchlorate concentrations for frogs collected from all sites. This is the first known published account of perchlorate-induced thyroid disruption in fish under field situations, only the second known published account for amphibians, and also points out the value of biomarkers for contaminant biomonitoring.