The physical and biological characteristics of five stations in and near a tidal creek were studied for 18 months. Tidal creeks and tidal rivers are differentiated and defined. Diel fluctuations of dissolved oxygen and temperature were greatest and tidal-induced salinity oscillations the least at the headwaters of the creek. Small salinity oscillations promoted a high overall diversity of fishes at the head-waters of the creek, even though few fish species maintained permanent populations there. Juvenile fishes were abundant in the summer and fall. Extreme temperature and low dissolved oxygen levels prompted a migration of fishes from these areas in late summer. Permanent fish populations were found at the mouth of the creek and in an adjacent tidal river.