Results are reported for chronic marine bioassays exposing the cyprinodontid fish Rivulus marmoratus, to 2,3,4,6-tetrachlorophenol. Purpose of the study was to illustrate the value of R. marmoratus as a chronic bioassay animal. Results demonstrated the relative insensitivity of R. marmoratus to chronic toxic effects of the chemical, but pathological changes were observed. Fish at least two-months-old showed gill damage ranging from severe (at the highest exposure concentration) to moderate and slight (at the lowest concentration). Such pathological changes might not be seen in chronic laboratory experiments with organisms of greater sensitivities.