Aim In this article the acute ecotoxicological effects of Cr(III), Cr(VI), Cd(II) and V(V) on ciliated protozoa isolated from Lake Maracaibo were evaluated, by estimating of the LC50 for an exposure time of 1-h and observations every 5 min. Methods Isolations and cultures of ciliated protozoa were made from surface water samples to then carry out toxicity essays under static and controlled conditions, identifying cell immobility (death) as the endpoint. Results The response of the ciliated protozoa made it possible to unequivocally determine the acute toxicity in presence of potentially toxic elements (PTE), with variable mortalities depending on the gender, the element tested and its concentration. The results obtained with Euplotes sp. indicate that protozoan is a sensitive biomonitor indicated for the biomonitoring of PTE contamination in Lake Maracaibo. Conclusions The use of shorter exposure periods offers opportunities to show early toxicity effects on natural populations and to act in a timely manner (early warning systems) in contamination events by PTEs, as well as the development of sensitive and rapid biomonitoring methods for detection of these elements in the environment.