Acute (96-h) toxicity tests were conducted using 30-day old fathead minnows (Pimephales promelas). A total of 139 chemicals were tested under flow-through conditions at a temperature of 25 plus or minus 1.0 degs C using Lake Superior water. A checklist was developed for systematically recording visual observations of behavioral and morphological changes. The checklist contained 40 different code items representing 10 general categories such as locomotor activity, morphological anomalies, etc. Loss of schooling behavior appeared to be the most sensitive indicator of general stress and was observed with 96% of the tested chemicals. Although deviate changes in behavior were found to be sensitive indicators of stress, they cannot be used solely to classify chemicals according to mode of action. Gross morphological changes are equally important. Development of a model, based on combinations of behavioral and morphological data, appears possible for screening and differentiating chemicals according to toxic action.