Laboratory algal bioassays using both cultures ofAnkistrodesmus bibraianusand natural phytoplankton, and large, in-lake, container assays with natural populations, were used to determine the factor most limiting phytoplankton production in Lake Chapala, México. Both types of laboratory culture assays showed that nitrogen was the principal limiting nutrient at each station across this very large lake in all seasons. The growth response of natural phytoplankton was similar to that ofA.bibraianus. However, management practices to regulate the lake's productivity based solely upon this laboratory information would be inappropriate because the natural population assays showed that the ultimate limiting factorinsituis illumination controlled by the high clay turbidity. Rarely, if ever, was phytoplankton production controlled by the laboratory-determined limiting nutrient, nitrogen, expressed in the lake. The importance of performing algal assays extensively through time and space also was demonstrated. The nitrogen-augmented increase over controls inA.bibraianusbiomass ranged from less than 50 percent at the end of the dry season to greater than 1,000 percent in the middle of the rainy season. The average annual percent response at different sampling stations ranged from 438 percent to 541 percent. Also, the sample to sample variation was different for different stations. The variation coefficient was only 35 percent for the mid-lake station, but greater than 84 percent for the station nearer the source of river inputs.