This paper describes a portable chamber that measures net primary production of stream periphyton using a 14C uptake method. The unique feature is that substrates are moved through water at a velocity of 20 cm s −1 rather than moving water over substrates. The chamber consists of a plexiglass cylinder that is 9 cm in height and 15 cm in diameter. On the top of the cylinder is a DC gearmotor powered by a 12 volt, deep cycle, marine battery. The motor turns a shaft that rotates a 13.3 cm plexiglass plate at a velocity of 20 cm s −1 . Small tiles (3.2 cm × 3.2 cm × 0.5 cm) that have natural algal assemblages are mounted on the rotating plate. After adding 500 ml of filtered stream water and 185 kBq (5 µCi) NaH14CO3 to the chamber, the chambers are placed along a stream margin for 5 h. Measurements of 14C uptake by algae on the tiles provide estimates of net primary production (NPP). To assess the sensitivity and practicality of the chamber, algal primary production was measured in open and closed canopy sections of Kingsley Creek, Randallsville, New York. In autumn, primary production was higher in the open than closed canopy section and NPP was lower in spring in both sections probably because of scouring of algae due to snowmelt.