Significant numbers of dead fish were reported in the river La Pasion in the Peten region in Guatemala during two instances in April and June 2015. Pesticide contamination or the depletion of oxygen by organic matter, alternatively a combination of both these factors, was put forward as a possible cause of the mortalities. The source of the pollution was believed to be overflowing at the oxidation basin of a nearby palm oil production plant. The events led to severe impacts on the livelihoods of approximately 15,000 people living in the riverine communities, dependant on La Pasion۪s waters for drinking, sanitation, cooking and fishing. Sampling and analysis of river water was undertaken in connection with the two events by national laboratories and showed traces of the pesticide malathion. To confirm the results, the Guatemalan Government requested support from the international community to investigate the immediate risks to human health due to residual contamination. Subsequently, two international experts were mobilized through the United Nations Environment Programme (UNEP) / Office for the Coordination of Humanitarian Affairs (OCHA) Joint Unit with support of the European Commission۪s Humanitarian Aid and Civil Protection department and the Swedish Civil Contingencies Agency to provide technical sampling and analysis advice to the national and local authorities.