This project evaluated an automated biological monitoring system that measures fish ventilatory responses (ventilatory rate, ventilatory depth, and cough rate) to detect developing toxic conditions in water. In laboratory tests, acutely toxic levels of brevetoxin (PbTx-2) and toxic Pfiesteria piscicida cultures caused large increases in cough rate. In the field, the automated biomonitoring system operated continuously for 3 mo on the Chicamacomico River, a tributary to the Chesapeake Bay with a history of intermittent toxic algal blooms. The field biomonitor identified five fish response events. Increased conductivity combined with a substantial decrease in water temperature probably caused one event, while contaminants probably surfactants released from inadequately rinsed particle filters produced another response. The other three events, characterized by greatly increased cough rate (two events) or increased ventilation rate and depth (one event), did not have identified causes. Water quality variations did not correspond to the timing of the three events, analyses of water taken by an automated sampler were negative for the presence of Pfiesteria or chemicals that could be associated with the observed responses, and no fish kills occurred on the Chicamacomico River during the monitoring period. Continuing activities to improve the biomonitoring system are discussed.