Phytoplankton are not distributed uniformly in their aquatic habitat. Vertical and horizontal distribution patterns of phytoplankton in a temperate pond ecosystem v/ere directly affected by an extensive duckweed mat. Light, temperature, and dissolved organic matter undoubtedly were important distribution determinors; dissolved oxygen, carbon dioxide, and pH were not. This study also suggests that two little considered components of light, day-length and quality are more important in phytoplankton distribution patterns than previously thought.