Presents results of an experiment conducted in artificial streams (flumes) in which the growth of periphyton (algae) was studied in relation to phosphorus concentrations in the water. The goal of the study was to determine whether periphyton growth in the Athabasca River was phosphorus limited and whether the degree of phosphorus limitation varied seasonally. The experimental flumes were located on site, adjacent to the Athabasca River at Hinton. The experiment was designed to test periphyton growth and biomass response to a gradient of phosphorus additions during fall and early spring. Periphyton response was compared for phosphorus additions of zero (control), 1, 10, and 25 micrograms per litre of phosphorus (spring and fall 1994) or zero, 0.1, 0.2, 1, 5, 10, 25, and 50 micrograms per litre of phosphorus (fall 1993). Results include relative specific growth rates as indicated by measuring the build-up of chlorophyll a, the level at which growth rate saturation occurred, and phosphorus concentrations required to reach maximum biomass levels.