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 1x 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 1x), 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.