The Chapala lake has been exposed to several sources and levels of contamination, like municipal discharges of different communities, waters of agricultural return and residual waters poured by industries, coming from the Lerma river. Therefore, it is important to characterize both, the conditions through time and the phytoplanktonics organisms in this aquatic system in every phase, as well as to analyze the variations of the water quality and their relationships in three areas of the lake. These activities were carried out every three months from May, 1992 to February, 1993, sampling the surface and to one meter deep in 21 stations in the Chapala lake. Later, a qualitative and quantitative analysis was carried of the phytoplanktonics organisms. The water quality was evaluated considering 23 parameters with data provided by the Centro de Estudios Limnológicos de Guadalajara, Jal. The higest phytoplanktonic abundance was of 16 048 cel/mL at the surface and to one meter deep of 8 309 cel/mL. Alogenics processes related to the different conditions presented by the phytoplankton structure were presented; turbidity was caused in a smaller scale by the algal quantity and affected directly the abundance of the organisms, the pH was basic and the nutrients like phosphorus a total match, ortophosphates and organic nitrogen were abundant, therefore no restrictive constraints were considered. Through the time the Chapala Lake has shown continuous values of quality of the water with fluctuations in some registrations but the eutrophic phase stays as a result of the indiscriminate contribution of pollutants of diverse types.