"Cadmium is a toxic metal for zooplankton that produces deformations. It is also considered an environmental hazard to aquatic life. Since it has a significant effect in some marine organisms, we used two native zooplankton species from Quintana Roo, Mexico to obtain data regarding cadmium toxicity including the threshold concentration for observable morphological alterations and the percentage of organisms with morphological alterations at the exposure concentrations. We used the rotifer Philodina cf roseola and the oligochaeta Aeolosoma hemprichi, since both feed from the algae Nannochloropsis oculata. Both animals were exposed to a cadmium concentration range from 0.05 mg/l (0.047 mg/l, real concentration) to 10.0 mg/l (9.39 mg/l, real concentration) for 24 h. The LC50 for cadmium in P. cf roseola was 0.7 mg/l (0.65 mg/l, real concentration), whereas in A. hemprichi was 3.38 mg/l (3.17 mg/l, real concentration). The exposure of cadmium at 0.5 mg/l (0.47 mg/l, real concentration) for less than 24 h induced morphological alterations in the lorica of rotifers, foot deformations, and constriction in the middle part of the body. In oligochaetes, damage to the epidermis was noted. The threshold concentration where deformations appeared in rotifer was 0.3 mg/l at 24 h, while in oligochaeta was 0.5 mg/l (0.47 mg/l, real concentration) at 30 min. We concluded that cadmium promotes alterations in aquatic organisms due to direct exposure added to the culture medium. "