Seasonal changes in physical, chemical, and biological features are described in four small lakes which show different types and degrees of meromixis. There is a wide range in total dissolved solid content between lakes as well as within lakes (surface to near-bottom): Yellow, 282 (surface) to 337 (bottom); White, 6229–7918; Mahoney, 10,003–86,906; Lyons, 12,116–222,195 mg/liter, but no marked seasonal changes. In Yellow Lake meromixis is probably maintained by morphometric features of the lake basin rather than a strong chemocline; in the other lakes complete mixing is inhibited by chemical density gradients, those in Mahoney and Lyons being especially sharp.Summer, autumn, winter (under ice), and spring vertical profiles are presented for temperature, dissolved oxygen, pH, alkalinity, specific conductivity; differences within and between lakes are discussed. Other physical–chemical characteristics are given in less detail.Purple sulphur bacteria occur in three of the lakes (White, Mahoney, Lyons) and in the latter two form discrete "plates" detectable by high frequency echo sounding (confirmed by sampling). Seasonal changes in depth of the bacterial plates are considered in relation to some physical and chemical parameters. Phytoplankton, zooplankton, and benthos communities are discussed in relation to the degree of meromixis shown by the lakes.