Valle de Bravo reservoir is used for aquatic, fishing and as a source of drinking water to Mexico City. Annual data on composition, abundances, species richness and diversity of the phytoplankton surface community and some physical-chemical parameters variations were discussed. Results showed a spatial homogeneity for environmental descriptors and phytoplankton samples but a temporal significant difference between months. Pulses of high algal densities corresponded to late stratification (October, 103 x 10(3) cell ml(-1)), early stratification (April, 107 x 10(3) cell ml(-1)) and plenty stratification (June, 69 x 10(3) cell ml(-1)). Taxa that reached higher densities were: Microcystis spp., Snowella septentrionalis, Anabaena spp., Aphanizomenon yezoense and Fragilaria crotonensis. Contribution of each taxon to the total phytoplankton density showed that majorities were rare (41%) or dominants (40%). Frequent alternation between pulses and low densities and diversity of phytoplankton as well as a relative high number of taxa found (68), could be explained by daily strong winds, unstable epilimnion thickness and incorporation and extraction of substantial volumes of water occurred in the reservoir. Dominances of cyanobacteria and some chlorococcal species and a high temporal fluctuated Shannon-Wiener diversity index (0.45- 2.35 bits) pointing to eutrophic and perturbed conditions.