Planktonic ciliates were examined during a spring-summer period (November 1998 to April 1999) in 2 South Andean lakes in Argentina, with different dissolved organic carbon (DOC) and nutrient concentrations. Lake Moreno Oeste is a deep warm monomictic lake (Zmax= 90 m), while Lake Morenito is shallower (Zmax = 12 m) and polymictic. Sampling was performed in a mid-lake station, where vertical profiles of temperature, light, and chlorophyll a concentration were measured in situ. In addition, DOC and phosphorus concentrations were determined, and samples for bacteria, phyto- plankton and ciliate enumeration were taken along the water columns. Lake Morenito presented DOC and phosphorus concentrations 3-fold higher than those of Lake Moreno Oeste. Two different ciliate assemblages were present. Lake Moreno Oeste was dominated by the large mixotrophic ciliate Ophry- dium naumanni, which was absent in Lake Morenito. In order to test 0. naumanni water dependence, a series of laboratory experiments were performed during the sampling season. The experiments con- sisted of the incubation of the ciliate in filtered water from both lakes and the subsequent analysis of growth rates in each treatment. In addition, experiments with Moreno Oeste lake water with a 4-fold increased phosphorus concentration (addition of MBL culture medium) were conducted. The incuba- tion of 0. naumanni in Lake Morenito water led to highly negative growth rates of the ciliate. Results showed that the addition of nutrients did not cause significant differences in the growth rates.