Cases of deaths in manure or septic tanks are rare in legal-medical practice, more frequently as unfortunate occupational accidents. Poisoning with toxic gases, especially with hydrogen sulfide, is reported as the cause of death, while the exhaustion of oxygen in the air is omitted with the simultaneous excess of carbon dioxide. In such cases, determination of the direct cause of death constitutes a big problem because post-mortem examination does not reveal the specific changes. A case of acute collective poisoning by gases in a manure storage tank is presented of 5 agricultural workers, 2 of whom died. While explaining the cause of poisoning and deaths, toxicological blood tests were performed in the victims of the accident, as well as gases inside the manure storage tank. The post-mortem examinations and toxicological blood tests performed did not allow determination of the direct cause of death. Toxicological tests of gases from inside the manure tank showed a very low concentration of oxygen, with a simultaneous very high concentration of carbon dioxide, and a considerable level of hydrogen sulfide. The cause of fainting of three and deaths of two workers was not the poisoning with hydrogen sulfide, but oxygen deficiency in the air of the tank.