Study on physicochemical properties and freshwater fish was conducted in Lake Duminagat, a lone small crater lake of Mount Malindang Natural Park with a total area of 8.04 hectares. Temperature, pH, oxidation reduction potential (ORP), electrical conductivity, turbidity, dissolved oxygen (DO), and total dissolved solids of (TDS) were the water parameters analyzed. The survey was undertaken in May 2013. The pH, turbidity, and DO in Lake Duminagat fall within the permissible limit for Class AA water body classification intended primarily for waters having watersheds that are declared as protected areas, whereas temperature and TDS were below the standards. Not all of the water parameters analyzed showed significant relationship with water depth. A total of six species of freshwater fish belonging to three families was collected. Diversity Index was found to be high (1.63). The most abundant species in the lake are the native Barbodes binotatus (34.43%) and the exotic Xiphophorus helleri (21.31%), whereas the native Anguilla marmorata (4.92%) was found to be the least abundant. Interestingly, three new exotic freshwater fish species were recorded, the Xiphophorus helleri, Xiphophorus maculatus, and Poecilia reticulata. This study provides an important update of the status of Lake Duminagat.