Monthly whole-water phytoplankton samples were collected in Lake Michigan near Michigan City, Indiana from June, 1973 to May, 1974 on three transects at eight stations with depths ranging from 5 to 18m.A total of 226 taxa were identified in composite samples. The diatoms Fragilaria crotorensis (16;6), Thalassiosira fluviatilis (11%), and Tabellaria fenestrata (95%), numerically dominated the Lake Michigan phytoplankton assemblage during the study period.Principal components ordination revealed a well defined pattern of seasonal succession. No consistent differences between stations were found. Species diversity indices generally indicated a high level of community organization in spring and fall months and lowered estimates for summer and winter months.The phytoplankton community was dominated by diatoms throughout the year. Green and blue-green forms showed greatest abundance in the summer and. early fall with only minor occurrences during the remainder of the sample period.Species encountered in this study indicate continued succession of Lake Michigan phytoplankton and eutrophication of the southern near-shore water.Ball State UniversityMuncie, IN 47306