Results of a nutrient addition bioassay to determine whether phosphorus or nitrogen was limiting phytoplankton (algae, plants) growth in Cutler Reservoir, Utah. Water was collected from two parts of the reservoir and separated into flasks. Some flasks received additions of phosphorus (phosphate), others nitrogen (ammonium nitrate), others both nutrients, and the Controls received nothing. After a 4-day incubation in the laboaratory it is clear that the phytoplankton in north Cutler Reservoir were limited by nitrogen, whereas those in the south needed both nitrogen and phosphorus to stimulate growth. Chlorophyll a analyses confirmed the visual impression of the responses.