Juvenile (14-16 cm total length) and adult (24-26 cm) smallmouth bass Micropterus dolomieui were studied in a laboratory stream tank to determine the influence of light, current velocity, substrate, and cover on habitat selection. These factors were varied singly and in combination for ranges of 30-4,000 lx light intensity; 1-47 and 0-20 cm?s current; substrates of sand (1 mm), gravel (15-40 mm), boulders with cobble filler (250 mm with 50-100 mm), and boulders with cobble removed; and three types of clear and opaque covers. Fish exhibited strong preferences for slow current (< 10 cm?s), low light (<300 lx), and cover that provided slow current and low light. Substrate was only important when fish could get into the interstices. Juveniles selected slightly slower current speeds and lower light intensities than adults, but were similar in all other respects. Within the geographic range of a fish species, availability of preferred habitat may explain variation in fish densities (Sale 1972; Hunt 1976; DeVore and White 1978). Habitat preferences are influenced by biotic and abiotic factors. Important in habitat selection by fishes (Butler and Hawthorne 1968; Moyle and Vondracek 1985), abiotic factors may be used to assess the quality of a given area or they may be strongly correlated with biotic factors such as food and protection from predators. Because smallmouth bass are important sportfish, their physical habitat requirements have been quantified in a general way. In flowing waters they inhabit clear-water streams with moderate to high gradients, rocky substrates, and pool-riffle combinations (Hubbs and Bailey 1938; Reynolds 1965; Coble 1975). Habitat choice by adult smallmouth bass has been examined in the field (Munther 1970; Paragamian and Coble 1975; Probst et al. 1984) • Present address: