The life histories of the mimic shiner (Notropis volucellus), the common shiner (N. cornutus), and the bluntnose minnow (Pimephales notatus), from a small Minnesota Lake, were compared in order to find out how temperate fish species segregate ecologically and how accurate dietary studies are for showing ecological differences among similar co-existing species. The three minnow species were found to differ from each other in habitat, feeding behavior, organisms consumed, population age structure, and breeding habits. Mimic shiners occurred in large schools in shallow water and fed during the day, with peaks at dawn and dusk, on emerging and flying insects. They usually spawned in August but did not spawn at all in some years. Bluntnose minnows were found in small schools both in shallow water and in silty bottomed clearings in the aquatic vegetation in deeper water. They fed continuously on bottom organisms and detritus and spawned in July. Common shiners were roving opportunists, feeding on whatever organisms were most abundant. Their numbers fluctuated considerably but they were usually less abundant than the other two species. They spawned in June. All three cyprinids were ecologically segregated from the noncyprinid species in the lake as well. It was found that dietary studies can be reasonably good indicators of ecological differences if enough samples are taken at a variety of times and locations.