Arsenic concentrations of 3 mg/L are known to be lethal to zooplankton. Yet, a species namely Paracyclops chiltoni (Copepoda: Cyclopoida), has been reported to inhabit arsenic-contaminated surface water containing up to 53.23 mgAs/L in central Mexico. To investigate the structure (abundance, body length, and sex ratio) of this community as well as any potential morphological adaptation, we collected both zooplankton and water samples over a 4 year period. We determined water temperature, pH, electrical conductivity, dissolved oxygen, and oxidation reduction potential. We also determined total arsenic concentration in water samples in the laboratory. Next, we identified Paracyclops chiltoni organisms and determined abundance, body length and sex ratio and compared Paracyclops chiltoni morphology to published literature. The concentration of arsenic in water ranged from 25.5 mg/L to 65.2 mg/L, ​​exceeding both the Mexican guideline for the protection of aquatic life (0.2 mg/L) and the lethal concentration of arsenic (3 mg/L). Water temperature ranged from 18.3 °C to 22.7 °C. Water pH values were between 6.7 and 7.1. Electrical conductivity ranged from 2.7 mS/cm to 6.6 mS/cm, which was slightly higher than typical values for freshwater. Dissolved oxygen ranged from 0.06 mg/L to 4.54 mg/L, while oxidation reduction potential ranged from 158 mV to 484.2 mV, indicating a suboxic-oxic system. Chloride concentrations ranged from 182.3 mg/L to 203.4 mg/L. In general, we found Paracyclops chiltoni’s abundances were low compared to common abundance of zooplankton in non-contaminated waters. Zooplankton abundance was lowest in rainy season (0.8 ind/L) but highest during dry season (53.6 ind/L). Sex ratios (female:male) were lowest (0.5:1) during rainy season, while they were highest (21:1) during dry season. In copepods, sex ratio is strongly inclined towards females. In regards to the body length, females were between 607 and 747 μm, while males were between 605-733 μm, showing no significant differences to commonly reported sizes for females (556-857 μm) and males (351-751 μm). No differences were found between the morphology of Paracyclops chiltoni living in arsenic contaminated water against the same species described in literature. Further studies are, however, needed to investigate possible genetic adaptations and bioconcentration and speciation of arsenic in the organisms. Paracyclops chiltoni abundance showed an inverse relationship to oxidation reduction potential, dissolved oxygen, and chloride. Results from this study contribute to the discovery of new aquatic metallotolerant organisms, inhabiting highly contaminated water in epicontinental waters.