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Unlike temperate regions, tropical ecosystems are characterized by high temperatures (>18 °C) all year, promoting blooms of cyanobacteria which often produce secondary metabolites toxic to zooplankton. Nabor Carillo and the Recreational Lake are part of the saline, Lake Texcoco, in Central Mexico which is filled nowadays with treated waste water. Both water bodies are dominated by Planktothrix, Anabaenopsis, Spirulina and Microcystis. In this study we present the concentration of microcystins in these waterbodies over an annual cycle. We also evaluated the chronic effects of cyanobacterial crude extracts from both lakes on two clones of the rotifer Brachionus calyciflorus, one from Nabor Carrillo Lake and the other from a canal in the shallow, Lake Xochimilco. The experiments on population growth were performed, beginning with 10 individuals per container for each of the following treatments: control (no crude extract), concentrated crude extract, and diluted crude extract (50:50) with moderately hard water and Chlorella vulgaris in a concentration of 0.5 × 106 cells ml−1. The cyanotoxin levels were measured using an ELISA test and ranged between 0.20 and 2.4 μg L−1 in the lake water. The results showed that the Recreational Lake extracts were more toxic, killing the rotifers in less than five days. The r values ranged from −1.74 to 0.48 in the presence of the crude extracts and 0.16 and 0.24 in the controls. The results have been discussed with emphasis on the importance of conducting regular studies to test ecotoxicological impacts of cyanobacterial blooms in tropical waters.We evaluated the concentration of microcystins in the saline Lake Texcoco over one year.Effects of crude extracts from cyanobacteria blooms on Brachionus calyciflorus were analyzed.The cyanotoxin levels in the lake over the year ranged between 0.20 and 2.4 μg L−1.Extracts from blooms in August were most toxic to the rotifer.