Arctodiaptomus dorsalis, originally described to be distributed within the Americas, has been documented to occupy a significant number of freshwater lakes in the Philippines. This paper reports the first record of A. dorsalis in Lake Lanao, an ancient lake located in the island of Mindanao. Based on its very high abundance, and positive correlation between the total body length of females with lake dissolved nitrates, phosphates and chlorophyll a concentration, A. dorsalis has clearly established itself in the lake ecosystem and has benefitted in the steady increase in the lake's nutrient load. Lake Lanao's endemic Tropodiaptomus gigantoviger may be considered as another calanoid species to be displaced by A. dorsalis given that a thorough examination of plankton samples collected from Lake Lanao have not yielded individuals of T. gigantoviger.